Design and Control of a DC Grid for Offshore Wind Farms Final 20121206

Design and Control of A DC Grid for Offshore

Wind Farms

By

Fujin Deng

A dissertation submitted to

The Faculty of Engineering, Science, and Medicine, Aalborg University

in partial fulfilment of the requirements for the degree of

Doctor of Philosophy

Department of Energy Technology

AALBORG UNIVERSITY Aalborg, Denmark 2012

ii

Thesis title:

Design and Control of A DC Grid for Offshore Wind Farms

Name of PhD student:

Fujin Deng

Name and title of supervisor and any other supervisors:

Prof. Zhe Chen

List of published papers:

Paper 1: Fujin Deng and Zhe Chen: “Low-voltage ride-through of variable speed

wind turbines with permanent magnet synchronous generator”. In Proc 35th

Annual Conference of Industrial Electronics, 2009, pp. 621-626.

Paper 2: Fujin Deng and Zhe Chen: “A new structure based on cascaded

multilevel converter for variable speed wind turbine”. In Proc. 36th

Annual

Conference on IEEE Industrial Electronics Society, 2010, pp. 3167-3172.

Paper 3: Fujin Deng and Zhe Chen: “Variable speed wind turbine based on

multiple generators drive-train configuration”. In Proc. IEEE PES Innovative

Smart Grid Technologies Conference, 2010, pp. 1-8.

Paper 4: Fujin Deng and Zhe Chen: “An offshore wind farm with DC grid

connection and its performance under power system transients”. In Proc. IEEE

Power and Energy Society General Meeting, July, 2011, pp.1-8.

Paper 5: Fujin Deng and Zhe Chen: “Control of improved full-bridge three-level

DC/DC converter for wind turbines in a DC grid”. IEEE Transaction on Power

Electronics, vol. 28, no. 1, pp. 214-324, January 2013.

Paper 6: Fujin Deng and Zhe Chen: “Design of protective inductors for HVDC

transmission line within DC grid offshore wind farms”. Accepted, IEEE

Transactions on Power Delivery.

iii

Paper 7: Fujin Deng and Zhe Chen. “Operation and control of a DC-grid offshore

wind farm under DC transmission system faults”. Submitted to IEEE

Transactions on Power Delivery, February 2012.

Paper 8: Fujin Deng and Zhe Chen: “A control method for voltage balancing in

modular multilevel converters”. Submitted to IEEE Transactions on Power

Electronics, August 2012.

This thesis has been submitted for assessment in partial fulfilment of the PhD degree.

The thesis is based on the submitted or published scientific papers which are listed above.

Parts of the papers are used directly or indirectly in the extended summary of the thesis.

As part of the assessment, co-author statements have been made available to the

assessment committee and are also available at the Faculty. The thesis is not in its present

form acceptable for open publication but only in limited and closed circulation as

copyright may not be ensured.

Abstract

iv

Abstract

Wind power is growing rapidly around the world, and the offshore wind farm is

currently seen as a promising solution to satisfy the growing demand for renewable

energy source. Along with the increase in the capacity of offshore wind farms and the

distance between offshore wind farms and land, the high-voltage direct current (HVDC)

is attractive. In addition, the DC grid may also be interested for interconnecting the wind

turbines in the collection level. As a consequence, a DC grid can be established for the

offshore wind farm, where the wind power collection system and power transmission

system both adopt DC technology. S far, the existing grid codes for wind turbines are

mainly focused on AC system. Therefore, the faults analysis in the DC grid and the

appropriate fault protections are required for the DC grid.

This thesis focuses on the design and control of the DC grid for offshore wind farms.

The DC grid layout for offshore wind farm is introduced. The wind turbine

configurations in DC grid are studied. A DC/DC converter is proposed for the wind

turbine directly integrating to the DC grid. The operation principle of the DC/DC

converter and the control of the wind turbine in DC grid are proposed as well. The HVDC

transmission system configuration is studied. A DC/DC converter is proposed as the

offshore converter to step up the collection level voltage to the transmission level voltage.

The control strategies for the offshore converter and the onshore converter are proposed.

The control of the DC grid under AC grid faults is discussed, and the improved control is

presented to ride-through the AC grid faults for the DC grid. The cable fault in the HVDC

transmission system is discussed. The performances of the HVDC system under faults are

analyzed. And then, the protective inductor is proposed and designed for the HVDC

system. Afterwards, the redundancy of the HVDC system under cable faults is studied,

and a fault ride-through strategy is proposed for the DC grid under cable faults.

Throughout the thesis, the DC grid for offshore wind farms examples are modeled

with the professional tool PSCAD/EMTDC, and the DC/DC converter prototype was

built and tested in the laboratory, and the results verify the theoretical analysis.

v Acknowledgements

Acknowledgements

The financial support for this research project given by the Department of Energy

Technology and the China Scholarship Council is gratefully acknowledged.

I would like to express my deepest gratitude to my supervisor Prof. Zhe Chen for all

support, guidance and encouragement during the project. I would also like to thank my

colleagues Zhou Liu and Yunqian Zhang for lots of help in life and work. Thanks also to

the people in the Department of Energy Technology for help during my PhD period.

Finally, I would like to thank my family and friends for their love and support.

Fujin Deng

Aalborg, Denmark

August, 2012

Contents

vi

Contents

Abstract i

Acknowledgements ii

Abbreviations vii

Contents iii

1. Introduction ............................................................................................................................... 1

1.1 Background ........................................................................................................................... 1

1.2 Overview of Previous Work .................................................................................................. 2

1.3 Aims and Main Contributions of the Thesis .......................................................................... 3

1.4 Layout of the Thesis .............................................................................................................. 4

1.5 Publications ........................................................................................................................... 5

2. DC Grids for Offshore Wind Farms ........................................................................................ 7

2.1 Introduction ........................................................................................................................... 7

2.2 DC-Grid Layout..................................................................................................................... 8

2.3 Wind Turbine....................................................................................................................... 10

2.3.1 Wind Turbine Overview ............................................................................................... 10

2.3.2 DC-Grid Wind Turbine ................................................................................................ 12

2.4 HVDC System ..................................................................................................................... 15

2.4.1 HVDC System Overview ............................................................................................. 15

2.4.2 HVDC System for DC Grid.......................................................................................... 17

2.5 Protective Devices ............................................................................................................... 17

2.6 Summary ............................................................................................................................. 19

3. Wind Turbine for DC Grids ................................................................................................... 20

vii Contents

3.1 Introduction ......................................................................................................................... 20

3.2 Wind Power Generator System ........................................................................................... 21

3.2.1 Aerodynamic System .................................................................................................... 21

3.2.2 Pitch Angle Control System ......................................................................................... 23

3.2.3 Mechanical Drive Train ................................................................................................ 24

3.2.4 Permanent Magnet Synchronous Generator ................................................................. 24

3.2.5 Wind Turbine I ............................................................................................................. 25

3.2.6 Wind Turbine II ............................................................................................................ 26

3.3 DC/DC Converter ................................................................................................................ 26

3.4 IFBTL DC/DC Converter .................................................................................................... 28

3.4.1 Converter Configuration ............................................................................................... 29

3.4.2 Modulation Strategy ..................................................................................................... 29

3.4.3 Voltage Balancing Control ........................................................................................... 31

3.5 Control of Wind Turbine in DC Grid .................................................................................. 32

3.5.1 Control of Wind Turbine I ............................................................................................ 32

3.5.2 Performance of Wind Turbine I .................................................................................... 34

3.5.3 Control of Wind Turbine II .......................................................................................... 37

3.5.4 Performance of Wind Turbine II .................................................................................. 37

3.6 Experimental Study ............................................................................................................ 40

3.7 Summary ............................................................................................................................. 46

4. Control of DC Grids for Offshore Wind Farms ................................................................... 47

4.1 Introduction ......................................................................................................................... 47

4.2 HVDC System Overview .................................................................................................... 48

4.3 DC Grid System .................................................................................................................. 51

4.4 Onshore Converter............................................................................................................... 52

4.4.1 Modular Multilevel Converter ...................................................................................... 53

4.4.2 Modulation Strategy ..................................................................................................... 55

4.4.3 Voltage Balancing Control ........................................................................................... 58

4.4.4 Circulating Current Elimination ................................................................................... 59

4.4.5 Onshore Converter Control .......................................................................................... 61

4.5 Offshore Converter .............................................................................................................. 63

4.5.1 Offshore Converter Configuration ................................................................................ 63

Contents

viii

4.5.2 Offshore Converter Control .......................................................................................... 63

4.6 AC Grid Faults Ride Through ............................................................................................. 65

4.6.1 AC Grid Faults ............................................................................................................. 65

4.6.2 Offshore Converter Control under Faults ..................................................................... 67

4.6.3 Wind Turbine Control under Faults .............................................................................. 68

4.6.4 DC Grid Performance under Faults .............................................................................. 70

4.7 Summary ............................................................................................................................. 73

5. Protection and Redundancy under HVDC System Faults ................................................... 74

5.1 Introduction ......................................................................................................................... 74

5.2 DC Cable Faults .................................................................................................................. 76

5.3 HVDC System Performance under Faults ........................................................................... 77

5.3.1 Dynamic Performance of Onshore Converter .............................................................. 77

5.3.2 Dynamic Performance of Offshore Converter .............................................................. 80

5.4 Protection for HVDC System .............................................................................................. 83

5.4.1 Protective Inductor for Onshore Converter .................................................................. 83

5.4.2 Dynamic Performance of Onshore Converter under Protection ................................... 85

5.4.3 Protection Inductor for Offshore Converter ................................................................. 88

5.4.4 Dynamic Performance of Offshore Converter under Protection .................................. 89

5.5 Redundancy of HVDC System ............................................................................................ 92

5.5.1 Onshore Converter Operation ....................................................................................... 92

5.5.2 Offshore Converter Operation ...................................................................................... 93

5.6 Fault Ride-Through Control for HVDC System ................................................................. 94

5.6.1 Onshore Converter Control .......................................................................................... 94

5.6.2 Offshore Converter Control .......................................................................................... 94

5.6.3 Chopper Resistor Control ............................................................................................. 95

5.6.4 Wind Turbine Control .................................................................................................. 95

5.7 System Performance under HVDC System Faults .............................................................. 97

5.8 Summary ........................................................................................................................... 101

6. Conclusion .............................................................................................................................. 102

6.1 Summary ........................................................................................................................... 102

6.2 Proposals for Future Work ................................................................................................ 104

ix

References .................................................................................................................................. 106

Appendix A 5 MW Wind Turbine System Parameters ........................................................ 116

Appendix B 2.5 MW Wind Turbine System Parameters ...................................................... 117

Appendix C Experimenal Circuit Parameters ....................................................................... 118

Appendix D Cable Parameters ................................................................................................ 119

Appendix E HVDC Transmission System Parameters ......................................................... 120

x

Abbreviation

AC Alternating current

CB Circuit breaker

DAB Dual active bridge

DC Direct current

DFIG Doubly fed induction generator

EMF Electric and magnetic field

EMI Electromagnetic interference

FSWT Fixed speed wind turbine

HB Half bridge

FB Full bridge

FBTL Full bridge three level

HVAC High voltage alternating current

HVDC High voltage direct current

IFBTL improved full-bridge three-level

IGBT Insulated gate bipolar transistor

kV Kilovollt

LCC Line Commutated converter

MFT Medium frequency transformer

MMC Modular multilevel converter

xi Abbreviation

MPPT Maximum power point tracking

NPC Neutral point clamped

NOFC Negative offshore converter

NONC Negative onshore converter

PCC Point of common coupling

PD Phase disposition

PMSG Permanent magnet synchronous generator

POFC Positive offshore converter

PONC Positive onshore converter

PS Phase shifted

PSC-SPWM Phase-shifted carrier-based sinusoidal pulse width modulation

PWM Pulse width modulation

SAB Single active bridge

SCIG Squirrel cage induction generator

SM Submodule

SPWM Sinusoidal pulse width modulation

VSC Voltage source converter

VSWT Variable speed wind turbine

WRIG Wound rotor induction generator

1 Introduction

Chapter 1

Introduction

1.1 Background

Wind power has been developed for a long time and kept the rapid growth around

the world [1]. All existing offshore wind farms have an AC collection system so far, the

collected power is then sent to an onshore AC grid through high-voltage alternating

current (HVAC) or HVDC transmission technology [2-4].

With the increase in the capacity of offshore wind farms and the distance between

the offshore wind farm and the land, the DC transmission, with the advantages such as

reactive power and harmonics and so on, becomes attractive under the growing trends of

the offshore wind farm development [5-8]. On the other hand, it may be an interesting

and cost-effective solution that the DC grid for the interconnection of the wind turbines in

the wind farm [9-15]. The DC grid only uses two cables which reduces the cost and the

loss. In addition, the DC/DC converters would be applied in the DC grid as the voltage

transformer instead of the large and heavy AC transformer. In DC grid, the output voltage

of the wind turbine will be boosted to the collection level voltage by a DC/DC converter,

and the collection level voltage will be further boosted to the transmission level voltage

by a DC/DC converter. Owing to the application of the medium frequency transformer in

the DC/DC converters, the size and weight of the component will be reduced, which is

very significant for the application of DC grid for the offshore wind farm [9-15].

Recently, the grid codes have been issued for the grid connected wind turbines or

wind farms to realize the continuity and security of transferring the wind power into the

grid. However, the wind turbines are collected with AC grid in all existing offshore wind

farms and the existing grid codes for the wind turbines are established for AC system,

and there are rarely grid codes published for wind turbines in DC grid [16-18]. The

Introduction

2

protective devices, such as DC circuit breakers, are still under development and not

commercially available [19-20]. As a consequence, many aspects related to DC grid need

to be studied, including the configuration and the control of the DC grid for offshore

wind farms under normal situation and cable faults situation, the redundancy of the DC

transmission system, and the operation of the DC grid after faults.

1.2 Overview of Previous Work

Regarding to the DC grid for offshore wind farms, the early solutions are mainly

focused on the HVDC transmission system, which is used to transmit the offshore wind

farm power to the land with the DC technology, especially for the large-scale offshore

wind farm and the long distance between the offshore wind farm and the land. Through

the HVDC technology, the loss and cost for the wind power system can be reduced [5-8].

Recently, the DC grid becomes attractive for the interconnection of the wind turbine

in the offshore wind farm, where the output of the wind turbine is DC instead of AC, and

the output of the wind turbine is directly connected into the DC grid.

The DC grid may offer some advantages for offshore wind farms in comparison with

the AC grid, such as costs, reactive power, and harmonics etc. Especially, the DC/DC

converter would be used in the DC grid instead of the ac transformer. In the DC grid, the

modern DC/DC converter with the medium frequency transformer would be used to step

up the voltage. Compared with the bulky 50 Hz transformer used in the traditional AC

grid, the frequency of the DC/DC converter in the DC grid is much higher than 50 Hz,

and the size and weight of the DC transformer can be reduced in comparison with the 50

Hz AC transformer. The size and weight of the device is very important in the

development of the offshore wind farm [9-15].

A few DC/DC converters for the offshore wind farm have been presented in

literatures [21-25], where the efficiency of the wind farm with different DC/DC

converters was evaluated. Further, based on the DC/DC converter, the layouts of the DC

grid for the offshore wind farm have been discussed. Besides, the basic control of the DC

grid for the offshore wind farm has been introduced as well.

The protection for the DC grid is an important issue, for example, the DC cable

faults would be severe and may damage the system, which becomes more challenging for

such a DC grid [26]. A few protection methods have been introduced, for example, the

DC switches and DC circuit breakers for isolating the faulty section of a DC system [27].

Especially, the passive and active DC breakers for high-voltage transmission system have

3 Introduction

been developed [28]. On the other hand, a few measures are presented to monitor the DC

fault and handle the DC faults in [26], [29-31].

As to the DC grid for offshore wind farms, further research is needed considering the

design and control of the wind turbine suitable for the DC grid. Although a few DC/DC

converters have been introduced before, there are still some challenges for the integration

of wind turbines into the DC grid. The HVDC transmission system should be studied for

the DC grid, which is obviously different from the existing offshore wind farms, because

the offshore converter in the DC grid is a DC/DC converter not an AC/DC converter as in

the existing HVDC transmission system. The design and control for the HVDC

transmission system in the DC grid is also an important issue. Although a few literatures

have reported the faults situation in the DC grid, the measures for the system protection is

not enough, for example, the DC circuit breaker is being developed and not suitable for

the perfect protection for the DC grid. Furthermore, the redundancy and the operation of

the DC grid under cable faults have been rarely reported in detail.

1.3 Aims and Main Contributions of the Thesis

Objective 1: To investigate the difference between the DC grid for offshore wind

farms and the existing offshore wind farm configurations, and the design and

control requirement for the DC grid

Contribution: The basic information of the DC grid for offshore wind farms

including layout, the important electrical components, protective devices, and

cable are discussed.

Objective 2: To develop the suitable wind turbine for DC grids

Contribution: The wind turbine for DC grid has been studied. A suitable DC/DC

converter associated with its operation principle applied for integrating the wind

turbine into the DC grid is presented. The control of the wind turbine in the DC

grid is also studied.

Objective 3: To develop the HVDC transmission system for DC grids

Contribution: The HVDC transmission system is developed in the DC grid for

the offshore wind farm, which is different from that in the existing wind farm

system. A DC/DC converter associated with its operation principle is presented

for the HVDC transmission system in the DC grid. The basic control of the

HVDC system in the DC grid is presented as well.

Introduction

4

Objective 4: To develop the control of the DC grid under AC grid faults

Contribution: The AC grid fault is studied. The control of the DC grid of

offshore wind farms to ride-through the AC grid fault is presented, which includes

the wind turbine control, offshore converter control, and the onshore converter

control. It has been proven that the wind farm can reconnect after the fault is

cleared within the time specified in existing grid codes.

Objective 5: To develop the protection for the HVDC system under cable faults

Contribution: The performance of the DC grid under cable faults has been

studied including the onshore converter and the offshore converter. The

corresponding protections for the onshore converter and the offshore converter are

presented and analyzed. The selection of the converter capacity and the protective

devices size is studied.

Aim 6: To develop the redundancy and the control for the DC grid under HVDC

transmission system faults

Contribution: The redundancy of the DC grid under the HVDC transmission

system fault is investigated, and the corresponding operation of the switchgears

for redundancy is studied. Furthermore, the control for the DC grid under HVDC

system faults is presented, which can effectively ride-through the HVDC system

faults for the DC grid.

1.4 Layout of the Thesis

The thesis studies the DC grid for offshore wind farms. Firstly, an overview of the

DC grid for offshore wind farms is presented in Chapter 2, where the system layout of the

DC grid is discussed. The main difference between the DC grid for offshore wind farm

and the existing offshore wind farm is investigated, and the main components in the DC

grid are indicated.

In Chapter 3, the variable-speed wind turbine (VSWT) for the DC grid is studied.

The improved full-bridge three-level isolated DC/DC converter is indicated as a possible

better option to integrate the wind turbine into the DC grid. In addition, the corresponding

modulation strategies, voltage balancing control, as well as the wind turbine control are

introduced. The simulation study and experimental test in the laboratory are both

conducted, and the results verify the feasibility of this wind turbine configuration.

In Chapter 4, the HVDC transmission system is investigated. The onshore converter

and the offshore converter associated with their control are discussed respectively. A

5 Introduction

DC/DC converter is studied and presented for the offshore DC/DC converter. The

performance of the DC grid under AC grid faults is analyzed, and the corresponding fault

ride-through control for the DC grid is presented.

The HVDC transmission system fault in the DC grid is investigated in Chapter 5.

The performances of the onshore converter and the offshore converter under faults are

analyzed, and the corresponding protections for the onshore converter and the offshore

converter are analyzed and designed. The redundancy for onshore station and offshore

station under HVDC system faults is presented, which can be realized with the actions of

the switchgears. The corresponding control for the wind turbine, the onshore converter,

and the offshore converter under HVDC system faults are presented, which can

effectively ride-through the HVDC system faults for the DC grid.

1.5 Publications

The publications are:

1. Fujin Deng and Zhe Chen, “Low-voltage ride-through of variable speed wind turbines

with permanent magnet synchronous generator,” in Proc 35th

Annual Conference of

Industrial Electronics, 2009, pp. 621-626.

2. Fujin Deng and Zhe Chen, “A new structure based on cascaded multilevel converter

for variable speed wind turbine,” in Proc. 36th

Annual Conference on IEEE Industrial

Electronics Society, 2010, pp. 3167-3172.

3. Fujin Deng and Zhe Chen, “Variable speed wind turbine based on multiple generators

drive-train configuration,” in Proc. IEEE PES Innovative Smart Grid Technologies

Conference, 2010, pp. 1-8.

4. Fujin Deng and Zhe Chen, “An offshore wind farm with DC grid connection and its

performance under power system transients,” in Proc. IEEE Power and Energy

Society General Meeting, July, 2011, pp.1-8.

5. Fujin Deng and Zhe Chen, “Control of improved full-bridge three-level DC/DC

converter for wind turbines in a DC grid,” IEEE Transaction on Power Electronics,

vol. 28, no. 1, pp. 214-324, January 2013.

Introduction

6

6. Fujin Deng and Zhe Chen, “Design of protective inductors for HVDC transmission

line within DC grid offshore wind farms,” accepted, IEEE Transactions on Power

Delivery.

7. Fujin Deng and Zhe Chen, “Operation and control of a DC-grid offshore wind farm

under DC transmission system faults,” submitted to IEEE Transactions on Power

Delivery, February 2012.

8. Fujin Deng and Zhe Chen, “A control method for voltage balancing in modular

multilevel converters,” submitted to IEEE Transactions on Power Electronics,

August 2012.

7 DC Grids for Offshore Wind Farms

Chapter 2

DC Grids for Offshore Wind Farms

2.1 Introduction

Along with the increase in the capacity of offshore wind farms and the distance

between the offshore wind farm and the land, the DC collection and the DC transmission

grid, with the advantages such as reactive power and harmonics etc., becomes attractive

under the growing trends of the offshore wind farm development [5-8].

Recently, the HVDC technology is being attractive for the long distance power

transmission of the large-scale offshore wind farm, which can effectively reduce the

cable loss and cost [5-8]. Owing to the benefits of the DC grid, the conception that using

DC grid for the interconnection of the wind turbine instead of AC collection grid in the

wind farm would be interesting for offshore wind farms [9-15]. As a consequence, the

DC grid for the offshore wind farm may be developed with the DC technology, where the

wind power in the wind farm are collected with DC network and transmitted to the

offshore station. And then, the wind farm power is sent to the on land grid with the

HVDC transmission system.

Similar to the existing offshore wind farm with AC collection system, the design and

control of the DC grid layout, and the key components and so on will be an important

issue, and should be considered.


8

2.2 DC-Grid Layout

The layout of the DC grid for offshore wind farms is still a matter of research and

discussion. A few DC-grid layouts have been indicated in some literatures as shown in

Fig. 2.1 [10]. In Fig. 2.1(a), a DC/DC converter is equipped in each wind turbine to step

up the DC-link voltage of the wind turbine to the medium voltage level for power

collection. And then, the collection voltage is boost up again by the DC/DC converter at

the offshore station to the transmission voltage level. In Fig. 2.1(b), only a DC/DC

converter is installed at the offshore station, which is used to step up the collection

voltage level to the transmission voltage level. In Fig. 2.1(c), the DC/DC converter at the

offshore station is omitted, and each wind turbine is equipped with a DC/DC converter to

step up its output voltage.

In Fig. 2.1(a), owing to the DC/DC converter is equipped in each wind turbine, the

collection voltage level can be set as a high voltage, which can reduce the cable losses at

the collection level. The DC-link voltage in the wind turbine can be controlled, which can

effectively increase reliability and improve system performance at the expense of the

additional DC/DC converter in the wind turbine. In Fig. 2.1(b), there is only one DC/DC

converter at the offshore station. The collection level voltage is the DC-link voltage of

the wind turbine, which is normally as low as a few kilovolts (kV). Especially, at the

large-scale offshore wind farm, the Fig. 2.1(b) may result in high current in the collection

level system, which may have a high requirement for the DC cable. As to Fig. 2.1(c),

where the DC/DC converter is only equipped in each wind turbine and the DC/DC

converter at the offshore station is removed, the DC/DC converter in each wind turbine

would step up the DC-link voltage of the wind turbine to the transmission voltage level,

which would have a high requirement for the wind turbine.

G

G

G

G

(a)


G

G

G

G

(b)

G

G

G

G

(c)

Fig. 2. 1. Block diagram of three DC-grid configurations for offshore wind farms. (a)

Configuration A. (b) Configuration B. (c) Configuration C.

Recently, the AC distribution and transmission are commonly used with mature

technologies. Hence, the DC grid for offshore wind farms can be designed in a similar

way as a wind farm with an AC grid, where the wind turbines are connected in radials to

a common DC/DC converter at the offshore station, and each DC cluster is in string

connection as shown in Fig. 2.2 [2-15]. In the DC grid, the AC output of the generator in

the wind turbine is converted into DC by the power converter and integrated into the DC

collection system. And then, the output power of the wind turbines is collected and

transferred to the offshore station, where the DC/DC converter is installed for stepping up

the voltage from the collection level to the transmission level. Afterwards, the wind farm

power is transmitted to the onshore converter with the HVDC technology and fed into the

AC grid.


10

onshoreoffshore

GridHVDC

T

offshore station

onshore station

WG WG

WG WG

Fig. 2.2. Block diagram of the DC grid for offshore wind farms.

2.3 Wind Turbine

As shown in Fig. 2.2, in order to achieve the DC grid for offshore wind farms, the

DC-grid wind turbine is an important component.

2.3.1 Wind Turbine Overview

With the rapid development of the wind power industry, the main three types of

typical generator systems including Squirrel cage induction generator (SCIG), doubly fed

induction generator (DFIG), and permanent magnet synchronous generator (PMSG), are

widely used for the wind turbine development. Owing to the difference of the rotation

speed range among the wind turbines, the wind turbine can be referred to as fixed speed,

limited variable speed, and variable speed [33].

Fixed Speed Concept

Fig. 2.3 shows the Fixed speed wind turbine (FSWT) system, where the SCIG stator

windings are directly connected to the grid through a transformer. A multiple-stage

gearbox is equipped in the FSWT system and the SCIG is only operated around the

synchronous speed. Therefore, the wind turbine, as shown in Fig. 2.3, is called fixed

speed wind turbine system. Normally, a capacitor bank is equipped for reactive power

compensation because the SCIG always draws reactive power from the grid. Although

there are a few advantages for the FSWT including robust, easy and cheap for mass

production and so on, a number of disadvantages also exist for the FSWT as follows [33].

The wind turbine speed is only in a narrow range


High mechanical and fatigue stress

High flicker

The grid voltage cannot be supported by this wind turbine system

Gearbox

Capacitor

Grid

SCIG

Fig. 2.3. Block diagram of the FSWT generation system.

Limited Variable Speed Concept

Fig. 2.4 shows the limited VSWT with a wound rotor induction generator (WRIG),

where the WRIG stator is directly connected to the grid through a transformer. The power

electronic converter is used to realize the variable rotor resistance. Through controlling

the power extracted from the WRIG, the variable speed operation can be realized [33].

WRIGGearbox

Converter

Grid

Fig. 2.4. Block diagram of limited VSWT generation system.

Variable Speed Concept with DFIG

Fig. 2.5 shows the VSWT based on DFIG, where the generator stator is directly

connected to the grid, and the generator rotor is connected to the grid through a power

electronic converter. The power converter controls the rotor speed. This type of wind

turbine can be operated in a wide speed range. Normally, the speed range of the DFIG-

based wind turbine is ±30% around the synchronous speed. The power converter is

approximately 25~30% of the wind turbine capacity. As a consequence, this type wind

turbine is attractive and popular from the view of economics. In addition, the power

electronic converter can compensate the reactive power and support the grid voltage.

However, the VSWT based on DFIG has the following disadvantages [33].

A multi-stage gearbox is necessary in the drive train

The slip ring may result in machine failure


12

Large stator current may be caused during grid faults owing to the stator is

directly connected to the grid

DFIGGearbox

Converter

Grid

Fig. 2.5. Block diagram of VSWT generation system based on DFIG.

Variable Speed Concept with PMSG

Fig. 2.6 shows the VSWT based on PMSG, where the direct-drive PMSG is

connected to the grid through a full-scale power converter. The generator is decoupled

from the grid by the power converter, which ensures that the grid disturbances have no

direct effect on the generator [33]. As a consequence, this type wind turbine can have a

good performance over the entire speed range.

PMSG Converter

Grid

Fig. 2.6. Block diagram of VSWT generation system based on PMSG.

2.3.2 DC-Grid Wind Turbine

Various wind turbines for the AC-grid integration have been developed and

commonly used with mature technologies, which is mainly consists of the generator and

the power converter. The DC-grid wind turbine can also be designed in a similar

construction.

Generator for DC-grid wind turbine

According to the aforementioned introduction, three main generator including SCIG,

DFIG, and the PMSG associated with different wind turbine configurations are popular in

wind power industry. With large-scale exploration and integration of wind sources,

VSWT system is becoming more popular than FSWT system [34], which results in that


the DFIG and PMSG may be interesting for the DC-grid wind turbine. On the other hand,

in comparison with the DFIG, the PMSG has a few advantages as follows [35].

High efficiency

High reliability such as no slip rings

Application of PM for rotor

Recently, the performance of the PMSG is being improved and the cost of the PM is

decreasing, which makes PMSG more attractive for the wind turbine. As a consequence,

the PMSG with a full-scale power converter will be a promising construction for the DC-

grid wind turbine.

AC/DC Converter for DC-Grid Wind Turbine

The power converter is a key component in the VSWT. So far, a few literatures

report the power converters for the VSWT based on PMSG, which are mainly contains

two types as shown in Fig. 2.7 [36-43]. In Fig. 2.7(a), the PMSG is connected to the DC

grid only with an AC/DC converter. In Fig. 2.7(b), the PMSG is connected to the DC grid

through an AC/DC converter and a DC/DC converter. In comparison with Fig. 2.7(a), a

grid-side DC/DC converter is used in Fig. 2.7(b).

PMSG

DC Grid

PMSG

DC Grid

(a) (b)

Fig. 2.7. Block diagram of DC-grid wind turbine. (a) Topology A. (b) Topology B.

So far, a few AC/DC converters and DC/DC converters have been reports for the

wind turbine generation system. Figs. 2.8(a) and (b) show the wind turbine configuration

based on the two-level and three-level voltage source converter (VSC) respectively,

which are widely used for the wind power generation system [36-40]. Fig. 2.8(c) shows

another wind turbine configuration with the diode rectifier and the boost converter [40-

43].


14

PMSG

P

N

PMSG

P

N

ab

c

(a) (b)

PMSG

P

N

(c)

Fig. 2.8. Block diagram of PMSG with (a) Two-level VSC. (b) Three-level VSC. (c)

Diode rectifier and boost converter.

Fig. 2.8(c) is the simplest configuration among the three types, which requires the

least number of switches and reduces the cost. However, the switch in Fig. 2.8(c) has to

take the full DC-link voltage, and the voltage change rate dv/dt is high. In addition, the

use of the diode rectifier as the generator side converter may cause high THD of the

generator output [44]. The two-level VSC, as shown in Fig. 2.8(a), is commonly used in

the wind turbine system because of its simple structure and few components. However,

along with the increase of the power and voltage range of the wind turbine, the two-level

VSC may suffer from larger switching losses and lower efficiency such as the MW and

MV power levels. In addition, the only two voltage stages cause higher dv/dt stresses to

the generator, and the bulky output filters may be required [45]. Three-level neutral point

clamped (NPC) topology, as shown in Fig. 2.8(b), is one of the most commercialized

multi-level converters for wind power systems. The switches in the three-level VSC only

take half of the DC-link voltage, which effectively reduces the dv/dt in comparison with

the other two configurations [45].

DC/DC Converter for DC-Grid Wind Turbine

The high power DC/DC converters are key components for realizing the DC-grid

wind turbine. In the DC grid, the DC/DC converters equipped with the medium


frequency transformer would replace the large and heavy AC transformers to boost the

output voltage of the wind generator to the medium voltage level for the collection

system. The application of DC/DC converters has a number of advantages since they can

eliminate the large AC transformers, and reduce the size and weight of the system.

To date, a number of DC/DC converters have been reported including isolated and

non-isolated configurations [21-25], [45-66], while most of the DC/DC converters are for

the low voltage and low power application, and the DC/DC converters for these high

power levels are still under development. The development of the DC/DC converter for

the DC grid will be an important issue.

2.4 HVDC System

2.4.1 HVDC System Overview

The HVDC technology is an efficient and flexible method to transmit large amounts

of electric power over long distances by overhead transmission lines or

underground/submarine cables.

Recently, there are mainly two types of HVDC transmission technologies. One is the

classical HVDC transmission system based on current source converters with naturally

commutated thyristors as shown in Fig. 2.9 (a), and called Line Commutated converter

(LCC) [2]. From Fig. 2.9(a), it can be seen that the classical HVDC transmission system

is naturally able to withstand short circuit currents due the DC inductors limiting the

current during faults condition. The other one is the new HVDC transmission using VSC

(Self-Commutated Voltage Source Converter) with pulse-width modulation as shown in

Figs. 2.9 (b)~(d) [3-5], [67].

These days, the VSC-HVDC transmission system becomes more and more attractive,

which are marked by ABB with the name “HVDC Light” and by Siemens with the name

“HVDC Plus”. In the VSC-HVDC technology, the Insulated gate bipolar transistors

(IGBTs) are used, and can switch off currents, which means that there is no need for an

active commutation voltage. The VSC-HVDC transmission does not requires a strong

offshore or onshore AC grid. Besides, the active and reactive power can be controlled

independently in the VSC-HVDC transmission system, which can reduce the need for

reactive power compensation and contribute to the stability of the AC grid. Furthermore,

owing to the use of IGBT semiconductors, the VSC-HVDC transmission system may

have a high switching frequency and reduce the harmonic content of the system, which

results in a reduced size of the filter required at the AC side.


16

A

B

C

P

N

Lt

A

B

C

P

N

LfC

C

a

bc

(a) (b)

A

B

C

Lf

P

N

C

C

ab

c

A

B

C

P

Lfa

SM

SM

SM

SM

SM

SM

b

SM

SM

SM

SM

SM

SM

c

SM

SM

SM

SM

SM

SMN

arm

+

Sub-module unit

Vsm

+

-

(c) (d)

Fig. 2.9. Block diagram of (a) Six-pulse valve LCC-HVDC. (b) Two-level VSC-HVDC.

(c) Three-level VSC-HVDC. (d) Modular multilevel converter (MMC)-based HVDC.

Normally, the DC/AC converter for the VSC-HVDC transmission system contains

two-level VSC, three-level VSC, and the MMC-based VSC as shown in Figs. 2.9(b)~(d).

Two- and Three-Level VSC for HVDC

As shown in Figs. 2.9(b) and (c), the two-level VSC is the simplest configuration and

only consists of six valves, which can create two voltage levels. The three-level topology

is currently a popular converter, which can generate three voltage levels. The two- and

three-level VSCs have been used in the commissioned projects [68], where the two- or

three-level pulse width modulation (PWM) are used to approximate the desired

waveform. For such converter topologies a high number of semiconductor devices with

blocking capability of several kilovolts are connected in series. In order to ensure uniform

voltage distribution, all devices connected in series in one converter arm must be

switched simultaneously. As a consequence, high and steep voltage steps are applied at

the AC side, which causes high component stresses and extensive filters are required.


MMC for HVDC

The MMC configuration is a novel multilevel configuration as shown in Fig. 2.9(d),

which has been introduced by Siemens into HVDC applications during recent years [69],

[70]. The MMC consists of six converter arms, each of which comprises a high number

of SMs and one converter reactor connected in series. The converter arm acts as a

controllable voltage source produced by a number of identical but individually

controllable SMs to create a high number of discrete voltage steps, which forms an

approximate sine wave with adjustable magnitude of the voltage at the AC terminal. Due

to the converter reactor in each arm, the effects of faults arising within or outside the

converter can be reduced [71]. As a consequence, the MMC technology provides

significant benefits for HVDC transmission systems.

2.4.2 HVDC System for DC Grid

As shown in Fig. 2.2, the HVDC transmission system contains an offshore station,

transmission cables, and an onshore station. At the offshore station, the offshore

converter is used to collect the DC clusters and convert the DC voltage from the

collection voltage level to the transmission voltage level. At the onshore station, the DC

power is converted into AC and sent into the AC grid. As a consequence, the HVDC

transmission system in the DC grid is different from the existing HVDC transmission

system, where the AC/DC or DC/AC power conversion is used at each point of the

HVDC transmission system. Hence, a HVDC transmission system configuration is

required for the DC grid as shown in Fig. 2.10, and a DC/DC converter should be

developed for the offshore converter in the DC grid.

DC/DC

Converter

DC/AC

Converter

Offshore station Onshore stationCable

Collection

level

Transmission

level

Fig. 2.10. Block diagram of HVDC transmission system in the DC grid.

2.5 Protective Devices

In the DC grid for offshore wind farms, the wind turbine, the offshore converter, and

the onshore converter are connected through submarine cables. In case of a cable fault, it

may take a long time to clear the fault. As a consequence, the protection in the DC grid


18

for offshore wind farms must be a priority. The protection of the offshore wind farm can

be done with AC circuit breakers, DC switches, and DC circuit breakers and so on. In

addition, the chopper resistor for discharging a DC link may be used for protection.

AC Circuit Breaker

The AC circuit breaker (CB) is widely used. It is the most economical way to protect

the DC system using the AC CB at the AC side. However, it requires a long time for the

AC CB to interrupt the circuit because of their mechanical restrictions, and the best

interrupting time for an AC CB is two cycles these days [72].

DC Switch

The DC switch is fast operating mechanical switch. However, the DC switch has no

capability of interrupting a fault current [30].

DC Circuit Breaker

The DC CB is different from the AC CB because of the absence of a natural current

zero crossing in the DC system. The DC CB has to interrupt short-circuit current very

quickly. Currently, the DC CB is only widely available for the low- and medium-voltage

range. The breaker interrupting HVDC short-circuit current is not commonly available

and has very limited ratings. A few proposals for the DC CB design have been presented

in the literatures, which comprise different series and parallel connections of classical AC

interrupters, resonance circuits with inductors and capacitors, semiconductors, charging

units, or resistors. For example, the passive and the active DC breaker based on standard

SF6 AC circuit breakers with auxiliary circuits for high-voltage transmission system has

been developed, which normally needs tens of milliseconds to interrupt the DC circuit

[27], [28]. Fig. 2.11 shows the circuit breaker with a passive auxiliary circuit. In the

circuit breaker with an active auxiliary circuit, the capacitor C is pre-charged prior to the

switching and one circuit breaker is inserted between capacitor C and inductor L [28].

CB

L C

idc

R

Fig. 2.11. Block diagram of a HVDC circuit breaker with a passive auxiliary circuit.


2.6 Summary

In this chapter, the DC grid for offshore wind farms is introduced, including its

layout, and the key components. The layout of the DC grid for offshore wind farms may

be similar to the AC-grid structure. The wind turbine configuration and the HVDC

transmission system configuration in the DC grid are different from that in the existing

AC offshore wind farm. In the DC grid, the DC/DC converter should be developed for

the DC integration of wind turbines and the HVDC transmission system.

Wind Turbine for DC Grids

20

Chapter 3


3.1 Introduction

The wind turbine is an important component in the DC grid, which is required to

produce a DC output and can be integrated into the DC grid directly. As introduced in

Chapter 2, a DC-grid wind turbine configuration is shown in Fig. 2.7(b), where the AC

output of the generator is converted into DC with the generator-side converter, and a

DC/DC converter is used as the grid-side converter to step up the DC-link voltage to a

high value. Two types of generator-side converter are introduced for the DC-grid wind

turbine. The first one is with the uncontrollable diode rectifier as the generator side

converter. The second one is with the VSC as the generator-side converter.

In the wind turbine with uncontrollable diode rectifier, the AC output of the

generator is converted to DC by the diode rectifier, and the generator is controlled by the

DC/DC converter. Although the wind turbine with the uncontrollable diode rectifier as

the generator-side converter has reduced cost in comparison with the wind turbine with

the VSC as the generator-side converter, the output THD of the generator with the diode

rectifier is much high in comparison with the generator with VSC. In the wind turbine

with the VSC as the generator-side converter, the generator is controlled with the VSC,

and the DC-link in the wind turbine is kept by the grid-side DC/DC converter, which can

reduce the disturbance of the DC grid on the wind turbine. In a word, a high efficient

DC/DC converter is required to realize the DC connection and power delivery for the

DC-grid wind turbine.

These days, most exist DC/DC converters are used in the low power applications.

The low power DC/DC converters are common but the high-voltage and high-power

21 Wind Turbine for DC Grids

DC/DC converters in the MW-level are rarely available on the market. So far, a lot of

DC/DC converters have been studied in literatures [45-67], such as boost converter,

forward converter, half-bridge (HF) converter, full-bridge (FB) converter, and Dual

active bridge (DAB) converter and so on. Their characteristics, performance, control, and

loss and so on are also compared as well. In addition, the three different topologies,

including the FB converter, the single active bridge (SAB) converter, and the resonant

converter, are evaluated for the applications in the DC grid for offshore wind farms,

regarding losses, contribution to energy production cost, design, and control and so on.

However, the three topologies are all the two-level configurations, which may be not

suitable for the medium-voltage and high-power application.

In this chapter, the possible DC/DC converters for wind turbine integrating into the

DC grid are considered and compared, and an isolated full-bridge three-level (IFBTL)

DC/DC converter is proposed to integrate the wind turbine into the DC grid. The

corresponding modulation strategy and control are also studied. Finally, a down-scaled

IFBTL DC/DC converter prototype was built and tested in the laboratory, and the results

verified the theoretical analysis.

3.2 Wind Power Generator System

3.2.1 Aerodynamic System

According to [36], the extracted wind power of the wind turbine can be described as

),(2

1 32pbw CvρπRP (3-

1)

with

ieCi

p

/4.1814.2 )2.13002.058.0151

(73.0

(3-2)

)1(

003.0

)02.0(

113

i

(3-

3)

where Pw is the extracted wind power, v is the wind speed, Rb is the blade radius, ρ is the

air density, CP is the power coefficient, θ is the pitch angle, λ=ωwR/v is the tip speed ratio,

ωw is the wind turbine speed.


22

According to (3-1)~(3-3), Fig. 3.1 shows the wind turbine power curves under

various wind speeds. There is a specific point in wind turbine power versus wind turbine

speed curve for each wind speed, where the wind turbine power is maximized [74]. As a

consequence, a variable-speed operation is used for the wind turbine, where the wind

turbine will follow the optimal power for the wind below the rated wind speed. When the

wind speed is high, the pitch angle control system will be activated to limit the wind

turbine at the rated power.

Fig. 3.1. Wind turbine power curves under different wind speeds.

Based on the aforementioned analysis, the optimal power captured from the wind can

be expressed as

3

3

max5

_2

1w

opt

p_

boptw ωλ

CρπRP (3-4)

Fig. 3.2 shows the power versus rotor speed characteristic of the wind turbine that

leads to optimal power capture from the wind based on the maximum power point

tracking (MPPT) method [75]. The 5 MW wind turbine parameters are used here as

shown in the Appendix A. The rotor speed of the wind turbine can be easily measured,

and normally used as a controller input instead of the wind speed to get the reference

power. A control curve AB is depicted in Fig. 3.2, because of the possible power

fluctuations when the rotor speed changes near the minimum rotor speed [73].


1

1

1.1Rotor speed [p.u.]

Po

wer

set

po

int

[p.u

.]

00.3 0.35

0.2

A

B

C

0

Fig. 3.2. Optimal power characteristic versus rotor speed.

3.2.2 Pitch Angle Control System

A pitch angle control system is normally needed to keep the turbine’s speed constant

without over-speed so as to limit the aerodynamic power of the wind turbine above the

rated wind speed. Owing to the big inertia constant (3 to 9 s) for the MW-level wind

turbine system, the pitch actuator angle in the MW-level wind turbine cannot be changed

very quickly. The maximum change rate of the pitch angle in a MW-level wind turbine is

normally between 3 and 10 degree per second [73].

Fig. 3.3 shows the pitch angle control system in the wind turbine, which is composed

with a controller and a first-order actuator. If the generator rotor speed ωe is below the

reference value ωmax, the wind turbine power is controlled to track the optimal power. If

the generator rotor speed ωe is over the ωmax, the pitch angle control system will be

activated to increase the pitch angle and drive the generator speed back to the maximum

permitted value [76].

+ )1

1(

sT

K T

1

s

1

-+

-

ωmax

ωeββref

T sKβ

sTβ

Fig. 3.3. Pitch angle control system.


24

3.2.3 Mechanical Drive Train

Fig. 3.4 shows the two-mass drive train, which has been proved to be suitable for

transient stability analysis [77]. According to [78], the dynamic of the two-mass drive

train can be described as

Wind turbine

GeneratorTw ωw K

D

Jw

Te ωe

Je

Fig. 3.4. Block diagram of the two-mass model.

eewwee

e

ewweww

w

TDKdt

dJ

DKTdt

dJ

)(

)(

(3-5)

where Tw and Te are the aerodynamic torque of the wind turbine and the generator torque,

respectively. Jw and Je are the equivalent wind turbine inertia and generator inertia,

respectively. ωw and ωe are the wind turbine and generator rotor speed, respectively. θwe

is the angle between the turbine rotor and the generator rotor. K is the shaft stiffness. D is

the shaft damping.

3.2.4 Permanent Magnet Synchronous Generator

The PMSG is chosen as the generator for the offshore wind farm in this study. In a

PMSG, the rotor excitation is constant. According to [79], [84], the PMSG model under

the synchronous reference frame can be described as


sq

sq

r

sq

sd

sq

sdesq

sq

sq

sd

sd

sq

sd

sq

esd

sd

sd

1)

1(

1

uL

ψL

iL

Lωi

L

R

dt

di

uL

iL

Lωi

L

R

dt

di

s

s

(3-6)

The electromagnetic torque can be expressed as

Te=1.5np((Ld-Lq)idiq+iqψf) (3-7)

where Rs is the stator resistance. Lsd and Lsq are the generator inductances in the dq

reference frame, respectively. usd and usq are the stator voltage in the dq reference frame,

respectively. isd and isq are the stator current in the dq reference frame, respectively. ωe is

the electrical rotating speed of the generator. Te is the electromagnetic torque. np is the

number of pole pairs. ψr is the rotor magnetic flux.

3.2.5 Wind Turbine I

Fig. 3.5 shows the configuration of the wind turbine I, where the three-level NPC

converter is used as the generator-side converter, and a DC/DC converter is used as the

grid-side converter. The PMSG is controlled by the NPC converter for the optimal power

capture, and the DC-link voltage VLV in the wind turbine is kept constant by the DC/DC

converter.

PMSG

ab

c

DC/DC

Converter

P

N

Collection

level

Vc1

Vc2

VLV+

+

+

-

M

Vo

+

-

io

Pg

Fig. 3.5. Block diagram of the wind turbine I.


26

3.2.6 Wind Turbine II

The configuration of the wind turbine II is shown in Fig. 3.6, where the diode

rectifier is used as the generator-side converter, and a DC/DC converter is used as the

grid-side converter. The diode rectifier convert the AC output of the generator to DC, and

the DC/DC converter boost the DC-link voltage to the collection voltage level. In wind

turbine configuration II, the PMSG is controlled by the DC/DC converter.

PMSGDC/DC

Converter

ii

Pg

ab

c

P

N

VLV

+

-

Collection

level

Vo

+

-

io

Fig. 3.6. Block diagram of the wind turbine II.

3.3 DC/DC Converter

In order to integrate the wind turbine into the DC grid, the high-efficiency DC/DC

converter is inevitable. A number of DC/DC converters have been presented in the

literatures [45-66]. In this thesis, the two-level and three-level configurations are mainly

considered, because both of the two configurations have been widely used for wind

turbines [66]. Among the existing DC/DC converters, a few possible candidates of the

DC/DC converters to integrate the wind turbine into the DC grid are shown in Fig. 3.7,

which mainly contains the basic FB two-level converter, the basic HF three-level

converter, the basic FB three-level converter, and the FB three-level converter based on

SMs. The four converter configurations for an example of a 2.5 MW wind turbine system

are considered in Table 3.1. The system parameters are shown in the Appendix B. The

rated input voltage Vi is 5.4 kV. The 1700V/600A IGBT (FZ600R17KE3) with the

nominal device voltage Vcom at 100 FIT as 900V is applied for the different converter

configurations [80].


C1+ Co +

Lo

Dr1 Dr3

Dr2 Dr4

s2

s1

s4

s3

Tr

Vi Vo

+

-

+

-

D1

D2

D3

D4

D1

D2

C1

C2

+

+

Co +

Lo

Dr1 Dr3

Dr2 Dr4s4

s1

D3

D4

D5

D6

Tr

Vi Vo

+

-

+

-

s3

s2

(a) (b)

D11

C1

C2

+

+

Co +

Lo

Dr1 Dr3

Dr2 Dr4D12

D5

Tr

Vi Vo

+

-

+

-

s8

D7

D8

s7

s5

s6 D6

D9

D10

D1

s4

D3

D4

s3

s1

s2 D2

Co +

Lo

Dr1 Dr3

Dr2 Dr4

Tr

+

SM

SM

SM

SM

SM

SM

SM

SM

D1s1

s2 D2

Csm

Vi Vo

Sub-module unit

Ls Ls

Ls Ls

+

-

+

-

Vsm

+

-

(c) (d)

Fig. 3.7. (a) Basic FB two-level converter. (b) Basic half-bridge (HB) three-level

converter. (c) Basic FB three-level converter. (d) Submodule (SMs) based FB Three-level

converter.

TABLE 3.1 POSSIBLE DC/DC CONVERTER OPTIONS

Converter topology Valve number

Switch number in each valve

Valve voltage

Produced voltage

level

Voltage balance control

complexity

Voltage sensor

2-level

Basic FB converter

4 6×1.7 kV IGBTs DC link voltage

2 No 1

3-

level

Basic HB

converter

4 3×1.7 kV IGBTs Half DC

link voltage

3 Easy 2

3-level

Basic FB converter

8 3×1.7 kV IGBTs Half DC link voltage

5 Easy 2

3-

level

FB converter

based on SMs

16 3 ×1.7 kV IGBTs

Half DC

link voltage

5 Calculation is

proportional to

the SMs number

8

Two-Level Converter

Fig. 3.7(a) shows the basic FB two-level converter, which is composed with four

IGBTs, a medium frequency transformer, a diode rectifier and an output inductor. In the

basic FB two-level converter, the input bridge produces a high frequency square wave at

the primary sides of the transformer. And then, the medium-frequency transformer (MFT)


28

transforms its second side voltage to a high level. Finally, the high voltage square wave at

the second side of the transformer is rectified by the diode rectifier. Owing to the output

inductor, the ripple of the current at the output side is effectively reduced.

From Fig. 3.7(a), it is easy to see that the required switches in the basic FB two-level

converter are only four, which is the least number among the four possible configurations.

However, as each switch in the basic FB two-level converter has to take the full DC link

voltage, the voltage change rate dv/dt is high, therefore, may cause larger electromagnetic

interference [55].

Three-Level Converter

Figs. 3.7(b)~(d) shows the possible three-level configurations for the DC/DC

converter, including the basic HF three-level converter, the basic FB three-level converter,

and the FB three-level converter based on SMs. The three-level converters have some

advantages in the aspects of power quality, and Electromagnetic interference (EMI) for

high power applications [55]. Furthermore, the switches in the basic HB three-level

converter, FB three-level converter and the SMs-based FB three-level converter only take

half of the DC link voltage, which effectively reduces the dv/dt in comparison with the

FB two-level converter.

On the other hand, In order to realize the N-level configuration, 8(N-1) switches are

needed for the SMs-based FB converter, whereas, the basic HB and FB converters only

requires 2(N-1) and 4(N-1) switches, respectively [49], [60]. In addition, the complicate

voltage balancing algorithm is required for the SMs-based FB converter [61].

The basic FBTL converter, with the advantage of the reduced voltage stress of the

switches, reduced filter size, and improved dynamic response, is becoming highly

suitable for medium-voltage and high-power conversion [49]. The basic FBTL and the

SMs-based FBTL configurations can both produce five-level output voltage and reduces

dv/dt in comparison with the basic HBTL configuration, particularly in the medium-

voltage and high power application [60], [81], [82]. However, the basic FBTL converter

has a simpler circuit structure and less number of switch devices than the SMs-based

FBTL configuration, which results in high reliability of the basic FBTL converter.

3.4 IFBTL DC/DC Converter

The FB converter may be a suitable choice for wind farm application from the view

of energy efficiency [25]. Hence, the isolated FBTL DC/DC converter is considered for

high-power wind turbine systems. The chopping phase-shift (CPS) control and double


phase-shift (DPS) control are presented in [50] and [51], respectively, for the isolated

FBTL converter, but they are not be suitable for the medium-voltage and high-power

system because of the high voltage change rate dv/dt. An IFBTL DC/DC converter is

presented for DC-grid wind turbine in this chapter, which may be used for high voltage

and high power system.

3.4.1 Converter Configuration

The IFBTL DC/DC converter configuration is shown in Fig. 3.8, which is mainly

composed with two divided capacitor, a three-level FB inverter, a passive filter, an MFT,

a diode rectifier, an output inductor, and an output capacitor. A passive filter is inserted

into the IFBTL DC/DC converter which can improve the performance of the DC/DC

converter [83]. As a consequence, the harmonics and the voltage stress of the MFT is

reduced, which is very significant for the power converter in the high-power application.

Ci1

Ci2

+

+

Co +

Ld

Dr1 Dr3

Dr2 Dr4

Vc1

Vc2

1:n

iLs Vt1

iLd

Vt2 Vd

ic1

ic2MFT

idr1 idr3

idr2 idr4

VoVi

ii

+

-

+

-

+

-

+ +

- -

D11

D12

D5

s8

D7

D8

s7

s5

s6 D6D9

D10

D1

s4

D3

D4

s3

s1

s2 D2

LsCs

a

b

it1 it2

io

Passive filter

Fig. 3.8. Block diagram of the IFBTL DC/DC converter.

3.4.2 Modulation Strategy

In the IFBTL DC/DC converter, the switches S1~S8 are switched complementarily

in pairs with a PWM (pair S1-S3, pair S4-S2, pair S5-S7, and pair S8-S6). The duty cycle

for S1 is D. Through phase-shift the PWM for the other switch pairs, the different

operation modes can be generated.

1) Operation Mode I: The operation mode I is shown in Fig. 3.9(a), where the PWM

waveform for the pairs of S8-S6, S5-S7 and S4-S2 lags behind that for the pair

S1-S3 by (D-Dc)Ts/2, Ts/2 and (D-Dc+1)Ts/2 respectively. Ts is the switching

cycle. The overlap time between S1-S3 and S8-S6 and between S4-S2 and S5-S7

are both DcTs/2. Dc is defined as the overlap duty ratio.


30

2) Operation Mode II: The operation mode I is shown in Fig. 3.9(b), where the

PWM waveform for the pair of S8-S6 leads before that for the pair S1-S3 by (D-

Dc)Ts/2, and the PWM waveform for the pair S4-S2 and S5-S7 lags behind that

for the pair S1-S3 by (1-D+Dc)Ts/2 and Ts/2 respectively. The overlap time

between S1-S3 and S8-S6, and between S4-S2 and S5-S7 are also both DcTs/2.

Vc1

Vc2

0

0

S10

0

1S4

0

1S5

0

1S8

1

ic1

ic2

Vi

iLs

DTs/2

DcTs/2

DTs/2

DTs/2

DTs/2

DcTs/2

C DA F G HB E A

Ts/2 Ts/2

0

0

0

Vab

0

Vi

Vi/2

-Vi/2-Vi

0Vt1

Vt2

0

it1

it2

0

iLdio

Vc2

Vc1

0

0

S10

0

1S4

0

1S5

0

1S8

1

ic1

ic2

Vi

iLs

DTs/2

DcTs/2

DTs/2

DTs/2

DTs/2

DcTs/2

C BA H G FD E A

Ts/2 Ts/2

0

0

0

Vab

0

Vi

Vi/2

-Vi/2-Vi

0Vt1

Vt2

0

it1

it2

0

iLdio

(a) (b)

Fig. 3.9. Performance of IFBTL DC/DC converter during one cycle. (a) In operation

mode I. (b) In operation mode II.


With the assumption Ci1=Ci2, the steady-state simulation results of the IFBTL

DC/DC converter under the proposed modulation strategies are shown in Fig. 3.9 in one

cycle Ts. The system parameters are given in Appendix C. The simulation results show

that the voltage Vab, Vt1, Vt2 and the current iLs, it1, it2 are all periodic waveforms with a

period of Ts, and the current ic1, ic2 and iLd are with a period of Ts/2. The performance of

the converter including the voltage Vt1, Vt2 and the current it1, it2 are improved with the

passive filter.

The main difference of the two operation modes is the capacitor charge and

discharge situation in each half cycle as shown in Fig. 3.9. The capacitor Ci2 discharge

more energy than the capacitor Ci1 in each half cycle in operation mode I, whereas the

capacitor Ci1 discharge more energy than the capacitor Ci2 in each half cycle in operation

mode II.

3.4.3 Voltage Balancing Control

A voltage balancing control strategy is proposed for the IFBTL DC/DC converter,

which can be realized by using the operation mode I and II.

In Operation Mode I

The capacitor current ic1 and ic2 are both with the period of Ts/2 as shown in Fig.

3.9(a). According to Fig. 3.9(a), the charge or discharge situations for the capacitors Ci1

and Ci2 in stages A, C and E are the same in the first half cycle. The period of the stage B

and D are the same. However, the current ic2 is more than ic1 in stage B, whereas the ic2 is

far less than ic1 in stage D. Suppose Vc1 = Vc2 = Vi/2, the Ci2 would provide more energy to

the load than the Ci1 in the first half cycle under the operation mode I. The analysis for

the first half cycle is the same to that in the first half cycle, and the situation in the second

half cycle is also the same to that in the first half cycles, which results in that the voltage

Vc1 would be increased and the voltage Vc2 would be reduced in operation mode I. As a

consequence, there will be the trend that the voltage Vc1 would be more than the Vc2 in

operation mode I as shown in Fig. 3.9(a).

In Operation Mode II

Liking operation mode I, the charge or discharge situations for the capacitors Ci1 and

Ci2 in stages A, C and E are the same in the first half cycle in operation mode II. The only

difference is that the current ic2 is less than ic1 in stage D, while the ic2 is far more than ic1

in stage B in operation mode II. The period of the stage B and D are the same. In stage D,

the current ic2 is less than ic1, while the ic2 is far more than ic1 in stage B. Suppose Vc1 =

Vc2 = Vi/2, the Ci1 would provide more energy to the load than the Ci2 in the first half


32

cycle under the operation mode II. The analysis for the second half cycle is the same to

that in the first half cycle, and the situation in the second half cycle is also the same to

that in the first half cycles, which results in that the voltage Vc2 would be increased and

the voltage Vc1 would be reduced in operation mode II. As a consequence, there will be

the trend that the voltage Vc2 would be more than the Vc1 in operation mode I as shown in

Fig. 3.9(b).

Voltage Balancing Control Strategy

From aforementioned analysis that the Vc1 would be more than Vc2 in operation mode

I, and Vc1 would be less than Vc2 in operation mode II, a capacitor voltage balancing

control strategy is proposed as shown in Fig. 3.10. A comparator is used to compare the

two capacitor voltage Vc1 and Vc2. When Vc1 is more than Vc2, the operation mode II will

be selected in the next half cycle. When Vc2 is more than Vc1, the operation mode I will be

selected in the next half cycle.

Operation mode

Vc1

Vc2

If Vc1≥Vc2 thenoperation mode II

Elseoperation mode I

Fig. 3.10. Block diagram of the proposed voltage balancing control for IFBTL converter.

3.5 Control of Wind Turbine in DC Grid

3.5.1 Control of Wind Turbine I

Fig. 3.5 shows the wind turbine I, where the three-level NPC converter is used as the

generator-side converter, and the IFBTL DC/DC converter is used as the grid-side

converter. In wind turbine I, the PMSG is controlled by the NPC converter for the

optimal power capture, and the DC-link voltage in the wind turbine is kept constant by

the IFBTL DC/DC converter.

Generator-Side Converter Control

The control structure for the generator-side converter is shown in Fig. 3.11, which is

based on the dynamic model of the PMSG in the synchronous rotating frame (3-6), with

the d-axis is aligned with the rotor flux [84]. As the converter is directly connected to the

PMSG, its q-axis current is proportional to the active power. The d-axis stator current is


proportional to the reactive power. The reactive power reference is set to zero to perform

unity power factor operation.

The double control loops structure is adopted for the control of the wind turbine I as

shown in Fig. 3.11. According to the measured generator speed ωr, the optimal power

command Pg_ref is calculated with the MPPT method based on the rotor speed versus

power characteristic as shown in Fig. 3.2 [85]. In the outside loop, a PI controller is used

to regulate the generator power Pg to track the reference Pg_ref, and produces

corresponding reference current iqs_ref. In inside loop, the PI controllers are used as the

current regulators to regulate d- and q-axis stator current to track the reference values.

PMSG

ia, icib,

dq

abcisd

isq

PWM

Generatordq

abc

Rotor Position

measurement

θed/dt

udcom = -Lsqωriqs

uqcom = Lsdωrisd - ωrψr

ua

ub

uc

udcom

uqcom

isd_ref=0

isd

+-

+ -

isqPg_ref

+-

Pg

++

++

ωr

C

+Vdc

isq_ref

PI

PIPI

Fig. 3.11. Block diagram of the control for the generator-side converter.

Grid-Side Converter Control

The grid-side converter, IFBTL DC/DC converter, is used here to keep the DC-link

voltage Vdc constant in the wind turbine. The control for the grid-side converter is shown

in Fig. 3.12, which adopts double-loop control structure. A simple PI controller is used as

the voltage regulator at the outside loop to generate the current reference iLd_ref, and the

other PI controller used as the current regulator is adopted in the inside loop to track the

current reference so as to follow the reference voltage Vdc_ref.


34

D

Improved

FBTL

Converter

Vdc

iLd-

+PI

Vdc_ref+ -

PI

DC

Grid

cable

cable

P

M

N

iLd_ref

Fig. 3.12. Block diagram of the control for the grid-side converter.

3.5.2 Performance of Wind Turbine I

The simulation study is conducted for the DC-grid wind turbine as shown in Fig. 3.5

with the professional tool PSCAD/EMTDC. The wind turbine parameters are shown in

the Appendix A.

The variable wind speed as shown in Fig. 3.13(a) is used to assess the performance

of the DC-grid wind turbine. Figs. 3.13(b) and (c) show the wind turbine speed and

power, respectively. With the proposed modulation strategy, voltage balancing control

and wind turbine control, the voltage Vdc is kept constant and the capacitor voltage Vc1

and Vc2 is controlled balanced as shown in Fig. 3.13(d). Figs. 3.13 (e) and (f) show the

output voltage Vo and current io of the DC-grid wind turbine, respectively.

(a) (b)


(c) (d)

(e) (f)

Fig. 3.13. (a) Wind speed. (b) Wind turbine speed. (c) Power Pg. (d) Voltage Vi, Vc1 and

Vc2. (e) Voltage Vo. (f) Current io.

In the wind turbine I, owing to the three-level configuration of the NPC PWM

converter, there will be the problem about the mid-point voltage balancing in the NPC

converter [45]. So far, a number of literatures have been reported about the control to

keep the mid-point voltage balancing in the NPC converter, where these control

algorithms are normally complicated [45]. As to the presented wind turbine I, an IFBTL

DC/DC converter associated with the voltage balancing control method is proposed,

which can effectively keep the mid-point voltage balancing in the NPC converter without

the complicated algorithm for the NPC converter.

Fig. 3.14 and Fig. 3.15 illustrate the simulation results about the capacitor voltage in

the different situations about wind turbine I, where a simple SPWM modulation control is

adopted. Fig. 3.14 shows the performance of the 3L-NPC converter, while the mid-point

M is not connected to the IFBTL DC/DC converter as shown in Fig. 3.5. In Fig. 3.14(a),

the DC-link voltage Vdc is kept constant. However, the capacitor voltage is not balanced

as shown in Fig. 3.14(b), because no voltage balancing control algorithm is used for the

three-level NPC converter. Fig. 3.15 shows the performance of the three-level NPC


36

converter in wind turbine I, where the mid-point M is connected to the IFBTL DC/DC

converter as shown in Fig. 3.5. Here, the DC-link voltage Vdc is kept constant as shown in

Fig. 15(a). Although there is no voltage balancing control for the three-level NPC

converter, a simple voltage balancing control algorithm is used for the IFBTL DC/DC

converter, which ensures the capacitor voltage balancing as shown in Fig. 15(b).

(a) (b)

Fig. 3.14. (a) DC-link voltage Vdc. (b) Capacitor voltage Vc1 and Vc2.

(a) (b)

Fig. 3.15. (a) DC-link voltage Vdc. (b) Capacitor voltage Vc1 and Vc2.


3.5.3 Control of Wind Turbine II

Fig. 3.16 shows the configuration of the wind turbine II, where the uncontrollable

diode rectifier is used as the generator-side converter and the IFBTL DC/DC converter is

used as the grid side converter. In the wind turbine II, the PMSG is controlled by the

IFBTL DC/DC converter to track the optimal power.

The control of the wind turbine II is shown in Fig. 3.16. The optimal power Pg_ref can

be obtained with the MPPT method [36] based on the measured wind turbine speed ωr.

Neglecting the power electronics losses, the power relationship can be presented as

Pg = Vi ii = Vo iLd (3-8)

where Pg is the generator power, ii is the input current of the IFBTL converter. A PI

controller is used for the power regulation at the outside loop and produces the current

reference iLd_ref. The other PI controller is used for the current regulation at the inside

loop and produces the duty cycle D for the IFBTL converter to follow the optimal power.

D

Variable speed

wind turbine

IFBTL

Converter

Vi

ii

iLdMPPT

Pg-

+ PIPg_ref + -

PI

Gearbox

PMSG

Cg

DC

Grid

cable

cable

ωr iLd_ref

Fig. 3.16. Block diagram of the control for the wind turbine II.

3.5.4 Performance of Wind Turbine II

The simulation study for the wind turbine II is also conducted with the professional

tool PSCAD/EMTDC. The wind turbine parameters are shown in the Appendix B. The

variable wind speed is shown in Fig. 3.17(a), which is used to assess the performance of

the wind turbine II. Fig. 3.17(b) shows the wind turbine speed. The wind turbine power is

shown in Fig. 3.17(c). Figs. 3.17(d) and (e) shows the DC-link voltage Vi, Vc1 and Vc2 and


38

the input current ii of the IFBTL converter, respectively, where the capacitor voltages are

kept balanced with the proposed control strategy. The voltage Vo and DC-grid current io

are shown in Figs. 3.17(f) and (g).

(a) (b)

(c) (d)

(e) (f)


(g)

Fig. 3.17. (a) Wind speed. (b) Wind turbine speed. (c) Power Pg. (d) Voltage Vi, Vc1 and

Vc2. (e) Current ii. (f) Voltage Vo. (g) Current io.

In order to realize the MPPT, the wind turbine speed should be varied in proportion

to the wind speed so as that the DC-grid wind turbine can capture the optimal power from

the wind. As a consequence, the generator power Pg and the DC-link voltage Vi should

follow the optimal curves [86]. With the simulation study of the DC-grid wind turbine,

the relationships between the optimal generator power Pg and the wind turbine speed ωr,

and between the optimal DC-link voltage Vi and the wind turbine speed ωr are shown in

Figs. 3.18(a) and (b), respectively.

(a) (b)

Fig. 3.18. (a) Optimal generator power Pg under the different wind turbine speed ωr. (b)

Optimal DC-link voltage Vi under the different wind turbine speed ωr.


40

3.6 Experimental Study

In the laboratory, a down-scale 1 kW IFBTL DC/DC converter prototype was built

and tested to verify the presented function of the IFBTL DC/DC converter. The switching

frequency is 5 kHz. The eight primary switches and diodes S1/D1-S8/D8 are the standard

IXTH30N25 power MOSFET. The clamping diodes D9-D12 are STTH3006. The

rectifier diodes Dr1-Dr4 are STTH3010. The transformer turn ratio is 1:2.6. The

transformer core is a PM87/70 ferrite core, and its leakage inductance is 10 μH. The filter

inductor Ls and capacitor Cs are 0.44 mH and 3 uF respectively. The inductor Ld is 0.8

mH. The capacitor Co is 1 mF. The input capacitors, both Ci1 and Ci2, are 400 V/330 μF.

The dead-time is set as 1.5 μs.

Fig. 3.19 shows the experimental circuit, where an isolated transformer is used

between the AC grid and the experiment system. A three-phase autotransformer followed

by a three-phase diode rectifier is employed at the input side to produce the input voltage

Vi. A DC power supply (SM300-10D) parallel with a resistor load Rload of 50 Ω at the

output side to emulate the DC grid and support the constant output voltage Vo as 250V.

An inductor with the value of 0.4 mH is inserted between the output of the converter and

the DC power supply.

In the experiment, the dSPACE equipment is used as the control system. The dS2004

is the AC/DC board, which is used to capture the signals including the voltage Vi, V1 and

the current iLd. And then, these captured signals are sent to the main processor, where the

voltage balancing control and the system control algorithms are conducted. Finally, the

PWM pulse signals s1~s8 are generated with the PWM pulse board dS1105 based on the

results from the processor.

Isolated

Transformer

AC Grid

Auto-

Transformer

ab

c+

Ci1

Ci2

Co

+

+

Vp Rload

LcLd

Ls

Cs

MFTVi

-

+

dS1004

s1+

-

iLd

dS1105

dS1006 Processpr

s1~s8

s2

s3

s4

s5

s6

s7

s8

dSPACE

Control System

V1

Fig. 3.19. Block diagram of the experimental circuit.


In order to verify the feasibility of the IFBTL DC/DC converter for wind turbines,

the two curves of the optimal power versus wind turbine speed and the optimal DC-link

voltage versus wind turbine speed as shown in Fig. 3.18 are used in the experiments,

where the power base is 1 kW and the voltage base is 180 V.

Experiment Study 1

The experiment study 1 is conducted with the experiment circuit as shown in Fig.

3.19, where the system power is set as 200 W. Fig. 3.20~ Fig. 3.23 shows the waveforms

of the system, where the (a) is with Dc/D=65%, and the (b) is with Dc/D=85%. Figs.

3.20(a) and (b) show the Vab, Vt1 and iLs, where the voltage Vab is five-level configuration,

which has symmetrical positive and negative segments. The Vt1 and iLs are effectively

improved by the passive filter as shown in Fig. 3.21. The voltage Vt1, Vt2, it1, it2, iLd and io

under the different Dc/D are nearly the same as shown in Fig. 3.21 and Fig. 3.22. The

capacitor voltage Vc1 and Vc2 are kept balanced with the proposed voltage balancing

control strategy as shown in Fig. 3.23.

(a) (b)

Fig. 3.20. Measured converter waveforms including Vab (100V/div), Vt1 (250V/div), and

iLs (10A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.


42

(a) (b)

Fig. 3.21. Measured converter waveforms including Vt1 (250V/div), Vt2 (250V/div), it1

(5A/div), and it2 (5A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.

(a) (b)

Fig. 3.22. Measured converter waveforms including Vo (100V/div), iLd (1A/div), and io

(1A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.


(a) (b)

Fig. 3.23. Measured converter waveforms including Vab (100V/div), Vc1 (100V/div), and

Vc2 (100V/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.

Experiment Study 2

The experiment study 2 is conducted with the experiment circuit as shown in Fig.

3.19, where the system power is 500 W. Fig. 3.24~Fig. 3.27 shows the experiment

waveforms of the system with Dc/D being 65% and 85% respectively. The Vt1 and iLs are

improved with the passive filter in the DC/DC converter as shown in Fig. 3.24. Fig. 3.25

and Fig. 3.26 shows the voltage Vt1, Vt2, it1, it2, iLd and io under the different Dc/D, which

are nearly the same. The capacitor voltage Vc1 and Vc2 are kept balanced with the

proposed control strategy as shown in Fig. 3.27.

(a) (b)

Fig. 3.24. Measured converter waveforms including Vab (250V/div), Vt1 (250V/div), and

iLs (10A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.


44

(a) (b)

Fig. 3.25. Measured converter waveforms including Vt1 (250V/div), Vt2 (250V/div), it1

(10A/div), and it2 (10A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.

(a) (b)

Fig. 3.26. Measured converter waveforms including Vo (100V/div), iLd (2A/div), and io

(2A/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.


(a) (b)

Fig. 3.27. Measured converter waveforms including Vab (100V/div), Vc1 (100V/div), and

Vc2 (100V/div). (a) Dc/D=65%. (b) Dc/D=85%. Time base is 40 μs/div.

Experiment Study 3

The dynamic of the IFBTL DC/DC converter is studied as well with the control

introduced in Fig. 3.16. According to the frequency-response design method [87], a

simple current PI controller was designed with Kp and Ki as 0.15 and 120, respectively. In

Fig. 28(a), the current io is stepped change from 0.4 A to 2.1 A in less than 4 ms. In Fig.

28(b), the current io is stepped change from 2.2 A to 0.5 A in less than 4 ms. Though the

output current was changed, there is no significant deviation between the capacitor

voltage Vc1 and Vc2 as shown in Fig. 3.28.

(a) (b)

Fig. 3.28. Measured converter waveforms including capacitor Ci1 voltage Vc1 (50V/div),

capacitor Ci2 voltage Vc2 (50V/div), and inductor current io (0.5A/div) under Dc/D = 65%.

(a) Inductor current io step up. (b) Inductor current io step down. Time base is 1 ms/div.


46

3.7 Summary

In this chapter, two DC-grid wind turbines are presented. One is with the three-level

NPC converter as the generator-side converter and the DC/DC converter as the grid-side

converter. The other one is with the diode rectifier as the generator-side converter and the

DC/DC converter as the grid-side converter. The corresponding control for the two types

of DC-grid wind turbines are introduced as well. The IFBTL DC/DC converter is

presented for the DC-grid wind turbine. With the passive filter, the voltage stress for the

medium frequency transformer can be reduced, which is significant for the high-voltage

and high-power system. In addition, with the IFBTL DC/DC converter, the voltage

balancing control for the generator-side converter (three-level NPC converter) can be

omitted, which can be realized by the grid-side converter (IFBTL DC/DC converter). As

a consequence, the system algorithm can be simplified. A down-scale IFBTL DC/DC

converter prototype was established in the laboratory and tested, and the results verify the

theoretical analysis.

47 Control of DC Grids for Offshore Wind Farms

Chapter 4

Control of DC Grids for Offshore Wind

Farms

4.1 Introduction

The HVDC transmission technology is widely used in the electric power system,

which is an efficient technology to transmit large amounts of electric power over long

distance by overhead transmission lines or underground or submarine cables [1-8].

In the existing offshore wind farm, the collection system is AC system, and the

offshore converter and onshore converter are both AC/DC converters. However, in the

DC grid for offshore wind farms, the DC collection grid is used instead of the AC

collection grid used in the existing offshore wind farm. The HVDC transmission

technology in the DC grid would be different from that in the existing offshore wind farm.

In the DC grid, the offshore converter is a DC/DC converter, which is used to step the

collection voltage level to the transmission voltage level. As a consequence, the converter

configurations associated with their control would be very important in the DC grid.

In this chapter, the HVDC transmission system in the DC grid is discussed in detail,

and the corresponding control is presented as well. Furthermore, the fault ride-through

control of the DC grid for offshore wind farms under AC grid faults is also introduced.

Control of DC Grids for Offshore Wind Farms

48

4.2 HVDC System Overview

According to the function and the location of the converter stations, various

configurations of the HVDC transmission system can be arranged as follows [88].

1) Monopolar HVDC system

The monopolar HVDC system configuration is shown in Fig. 4.1, where two

converters are used and separated by a single pole line, and a positive or a negative DC

voltage is used. The Monopolar HVDC system is used for many cable transmissions with

submarine connections, where there is only one insulated transmission conductor

installed and the ground or sea provides the path for the return current [88].

Fig. 4.1. Block diagram of the monopolar HVDC system.

2) Bipolar HVDC system

The bipolar HVDC system configuration is shown in Fig. 4.2, which increases the

power transfer capacity and is the most commonly used. Two insulated conductors as

positive and negative poles are used in the bipolar HVDC system, which can be operated

independently. In case of failure of one pole, the power transmission in the bipolar

HVDC system can continue in the other pole, which effectively increases the reliability

[88].


Fig. 4.2. Block diagram of the bipolar HVDC system.

3) Homopolar HVDC system

The homopolar HVDC system configuration is shown in Fig. 4.3, where two or more

conductors have the negative polarity and the ground or a metallic is used for current

return. The two poles operation in parallel in the homopolar HVDC system can

effectively reduce the insulation costs [88].

Fig. 4.3. Block diagram of the homopolar HVDC system.

4) Back-to-back HVDC system

The back-to-back HVDC system configuration is shown in Fig. 4.4, which is

commonly used to connect two AC systems [88].


50

Fig. 4.4. Block diagram of the back-to-back HVDC system.

5) Multiterminal HVDC system

The multiterminal HVDC system configuration is shown in Fig. 4.5, where a number

of HVDC converter stations are geographically separated and connected through

transmission lines or cables. In the multiterminal HVDC system, these converters can be

connected in parallel or in series [88].

(a)

(b)

Fig. 4.5. Block diagram of the multiterminal HVDC system. (a) Parallel

configuration. (b) Series configuration.


4.3 DC Grid System

The studied DC grid system for the offshore wind farm is shown in Fig. 4.6. The

DC-grid wind turbine I is considered in the DC grid, where the power generator is a

PMSG, the generator-side converter is a three-level NPC converter, and the grid-side

converter is an IFBTL DC/DC converter. The DC-grid wind turbines in the wind farm are

collected with the DC distribution system. Owing to the high voltage of the collection

system, the cable losses at the collection system can be effectively reduced. The DC

voltage in the collection level is stepped up to the transmission voltage level by the

offshore converter. And then, the wind farm power is sent to the land with the HVDC

technology and injected into the grid through the transformer.

In Fig. 4.6, the HVDC transmission system contains the offshore station,

transmission cables, and onshore station. The offshore converter and the onshore

converter are composed with positive onshore converter (PONC), negative onshore

converter (NONC), positive offshore converter (POFC), and negative offshore converter

(NOFC) respectively. The offshore converter is used to collect the DC clusters and

convert the DC voltage level from the collection voltage level to the transmission voltage

level. The onshore converter is used to convert the DC voltage into AC and sends the

wind farm power to the AC grid. Therefore, the HVDC transmission system in the DC

grid is different from the existing HVDC transmission system, where the existing HVDC

transmission system is used for AC/DC or DC/AC power conversion at each point of the

HVDC transmission system. As a consequence, a novel HVDC transmission system

configuration is required for the DC grid.

Grid

Collection

system

HVDC system

DC Cable VHV

offshore

station

PMSG

VLV

PMSG

VLV

PMSG

VLV

PMSG

VLV

VMV

PCC

VPCC

Vf1

Vf2

+-

+

-

POFC

NOFCoffshore

station

PONC

NONC

+-

Vn1

+

-Vn2

Fig. 4.6. Block diagram of the DC grid for the offshore wind farm.


52

4.4 Onshore Converter

MMC is an emerging and highly attractive topology for medium- or high-voltage

applications based on the features of modularity and scalability, particularly for HVDC

transmission system, with the advantages of small harmonics and small filter in

comparison with the two-level VSC topology [89].

The MMC was developed in the early 2000s, which is based on cascaded modular

cells. The MMC consist of a number of SMs, which is composed of half-bridge converter

cells fed by floating DC capacitors. There is no need for high-voltage DC-link capacitors

(or series connected capacitor) because the intrinsic capacitors of the cells perform these

tasks, which reduces the costs of the converter and increases its reliability. Besides, the

high number of levels enables a great reduction in the device average switching

frequency without compromise of power quality [90-94]. Furthermore, an inductor is in

series with the distributed energy storage capacitors in each arm, so the effects of faults

arising inside or outside the converter can be reduced substantially by the arm inductors

[95]. Owing to the aforementioned advantages, the MMC may be selected as the onshore

converter.

To date, the MMC has been reported in some literatures. The capacitor voltage-

balancing control is introduced in [96], where the PWM scheme is applied to MMC

based on the combination of averaging and balancing control without any external circuit,

and the results are verified by simulation and experiment. The dynamic performance of

an MMC-based, back-to-back HVDC system under balanced and unbalanced grid

condition is investigated in [61], where a phase-disposition (PD) sinusoidal pulse width

modulation (SPWM) strategy, including a voltage balancing method, for the operation of

an MMC is also presented. The performance of MMC operated with various multicarrier

SPWM techniques is evaluated in [97]. A new PWM modulation scheme is introduced

for MMC, and the semiconductor losses and loss distribution is investigated in [98]. An

efficient modeling method of MMC for electromagnetic transient simulation is proposed

in [99]. A reduced switching-frequency voltage balancing algorithms is developed for

MMC, and a circulating current suppressing controller is proposed for the three-phase

MMC in [100]. The impact of sampling frequency on harmonic distortion for MMC is

investigated in [101]. The inner energy control is discussed in [102].


4.4.1 Modular Multilevel Converter

The diagram of a three-phase MMC for onshore converter is shown in Fig. 4.7(a),

which is composed with six arms. Each arm consists of n SMs in series, and an arm

inductor Ls. The SM is depicted in Fig. 4.7(b), which contains an IGBT half bridge as

switching element and a DC storage capacitor with corresponding voltage as Vc. The

switches S1 and S2 in SM are controlled with complementary signals and produce two

active switching states that can connect or bypass its respective capacitor to the converter

leg. As a consequence, the output voltage Vsm of the SM can be decided depending on the

switching states, which is shown in Table 4.1. One thing mentioned is that, the SM state

is defined as switched-on if S1 is switch on and S2 is switch off. Here, the output voltage

Vsm equals to Vc. In contrast, the SM state is defined as switched-off if S1 is switched off

and S2 is switched on. In this situation, the Vsm is zero.

P

N

idc

ia

ib

Ls

Ls

Ls

Ls

iua iub

ila ilb

uua

uua1

uua2

uuan

ucua1

ucua2

ucuan

ucub1

ucub2

ucubn

ula

ula1

ula2

ulan

ucla1

ucla2

uclan

ulb

ulb1

ulb2

ulbn

uclb1

uclb2

uclbn

uub

O

uub1

uub2

uubn

Phase a Phase b

arm3

arm4

arm1

arm2

a

bic

Ls

Ls

iuc

ilc

ucuc1

ucuc2

ucucn

ulc

ulc1

ulc2

ulcn

uclc1

uclc2

uclcn

uuc

uuc1

uuc2

uucn

Phase c

arm5

arm6

c

RfLf

Vdc/2

Vdc/2

A

B

C

Cellua1

Cellua2

Celluan

Cellla1

Cellla2

Celllan

Cellub1

Cellub2

Cellubn

Celllb1

Celllb2

Celllbn

Celluc1

Celluc2

Cellucn

Celllc1

Celllc2

Celllcn

(a)


54

s1

s2

Vc

Vsm

+C

+

-

(b)

Fig. 4.7. (a) Diagram of three-phase MMC. (b) SM unit.

Table 4.1

SM state.

SM state S1 S2 Vsm

On On Off Vc

Off Off On 0

According to [99], the arm current in Fig. 4.7 can be expressed as

jdiff

j

lj

jdiff

j

uj

ii

i

ii

i

_

_

2

2 , cbaj ,,

(4-1)

where iuj and ilj are the upper arm current and the lower arm current of phase j (j=a, b, c),

respectively. ij is the AC current of phase j (j=a, b, c). idiff_j is the inner difference current

of phase j (j=a, b, c), respectively, which contains two parts as (4-2). One part is the DC

component, which is one-third of the DC-link current as idc/3. The other part is the

circulating current i2f_j of phase j (j=a, b, c).

jfdc

jdiff ii

i _2_3 , cbaj ,,

(4-2)

and

0_2_2_2 cfbfaf iii

(4-3)


Although the circulating currents i2f_j (j=a, b, c) flow through the three phases of the

MMC and have no effect on the AC side and the DC-link, it increases the rating values of

the MMC devices, the converter power losses, and the SM capacitor voltage ripple [99].

According to [99], the voltage relationship of the MMC in Fig. 1 can be described as

)()()( ljujs

ljuj

sljujdc iiRdt

di

dt

diLuuV , cbaj ,, (4-4)

where Vdc is the DC bus voltage. uuj and ulj are the total output voltage of the up and the

low series-connected SMs of phase j (j=a, b, c) respectively. Considering the point “o” as

shown in Fig. 4.7 as the fictitious DC-link mid-point, the AC side can be described as

[61], [99]

dt

diLi

Reu

jsj

sjjo

22 , cbaj ,, (4-5)

where ujo is the output voltage of phase j (j=a, b, c), ej=(ulj-ulj)/2 is the inner Electric and

magnetic field (EMF) generated in phase j (j=a, b, c).

4.4.2 Modulation Strategy

The modulation strategies for the MMC are mainly focused on the PD-SPWM

modulation strategy and the PSC-SPWM modulation strategy as follows.

1) PD-SPWM modulation strategy

Suppose that the capacitor voltage in each SM is the same with Vc=Vdc/n, in the

MMC with n SMs in each arm, the PD-SPWM modulation strategy can be used to

synthesize an (n+1)-level waveform at the AC-side of the MMC, where n in-phase carrier

waves displaced symmetrically with respect to the zero-axis. The output voltage level at

the AC-side of the MMC can be determined by comparing a sinusoidal reference wave

with the n carrier waves [61].

In each phase of the MMC, the total number of the SMs to be turned on at each

instant is n/2, which means that the summation of the number nu of the SMs turned on in

the upper arm and the number nl of the SMs turned on in the lower arm is n/2. The

numbers nu and nl can be determined by the desired voltage level of the phase from the

PD-SPWM modulation strategy.


56

Suppose that there are 4 SMs in each arm, and the capacitor voltage in each SM is

the same, an example of 5-level waveform at the AC side of the phase with respect to the

mid-point “o” can be synthesized as shown in Fig. 4.8.

(a)

(b)

Fig. 4.8. PD-SPWM modulation strategy. (a) Reference and carrier waves. (b) Produced

voltage level.

(1) Level 1: the output voltage ujo=-Vdc/2 (j=a, b, c). In this situation, 4 SMs in the

upper arm are turned on, and 4 SMs in the lower arm are turned off. In this situation,

nu=4, nl=0.

(2) Level 2: the output voltage ujo=-Vdc/4 (j=a, b, c). Here, there are 3 SMs turned on

in the upper arm, and only 1 SM turned on in the lower arm. Here, nu=3, nl=1.


(3) Level 3: the output voltage ujo=0 (j=a, b, c). Here, 2 SMs in the upper arm are

turned on, and 2 SMs in the lower arm is turned on. In this situation, nu=2, nl=2.

(4) Level 4: the output voltage ujo= Vdc/4 (j=a, b, c). Here, 1 SM in the upper arm is

turned on, and 3 SMs in the lower arm are turned on. Here, nu=1, nl=3.

(5) Level 5: the output voltage ujo= Vdc/2 (j=a, b, c). In this situation, 4 SMs in the

upper arm are turned off, and all the SMs in the lower arm are turned on. Here, nu=0, nl=4.

2) PSC-SPWM modulation strategy

The phase-shifted carrier-based SPWM modulation can also be used for the MMC

with the advantage that it can suppress some harmonics for multilevel converters,

decrease the switching frequency, and reduce the losses [99].

The PSC-SPWM modulation strategy is shown in Fig. 4.9(a), where each arm with n

SMs requires n carrier waves, and these carriers are phase shifted by 360o/n. By

comparing the reference wave and the n carrier waves, the total number non of SMs that

are needed to be switched on simultaneously in each arm can be decided as shown in Fig.

4.9(b), which would be used in the voltage balancing control described later.

(a)

(b)


58

Fig. 4.9 PS-SPWM modulation strategy. (a) Reference and carrier waveforms. (b)

Number of switching on.

The above description introduces the number of the SM in the upper and the lower

arms to be switched on with PD-SPWM and PSC-SPWM, respectively. As to the specific

SM to be turned on, it will be introduced later in the voltage balancing control.

4.4.3 Voltage Balancing Control

The capacitor voltage control is an important issue, which should be monitored in

real time and kept balanced. In the aforementioned two modulation strategies, the number

of the SMs to be turned on at a time in the upper arm and the lower arm has been decided.

The capacitor voltage values and the direction of the arm current are used to select the

specific SMs to be switched on.

The capacitor voltage in each SM is related to its switch stage and the direction of

arm current as shown in Table 4.2. Normally, when the arm current is positive as shown

in Fig. 4.7, and the SM is turned on, the corresponding capacitor in the SM will be

charged and its voltage will be increased. When the arm current is negative and the SM in

the arm is turned on, the corresponding capacitor in the SM will be discharged and its

voltage will be decreased. No matter of the direction of the arm current, if the SM is

turned off, which means that the corresponding capacitor in the SM will be bypassed, and

the capacitor voltage will be unchanged.

In order to realize the capacitor voltage balancing control, the capacitor voltage of

the SMs in each arm are measured and sorted in descending order. The specific voltage

balancing control for SM can be presented as follows [61].

1) If the arm current is positive, the non SMs in the arm with the lowest voltage are

switched on.

2) If the arm current is negative, the non SMs in the arm with the highest voltage are

switched on.


TABLE 4.2

CAPACITOR STATE

Arm current

(iu or il)

SM

state

Capacitor

state

Capacitor voltage

Vc

Positive On Charge Increased

Off Bypass Unchanged

Negative On Discharge Decreased

Off Bypass Unchanged

4.4.4 Circulating Current Elimination

According to the analysis before, a circulating current would flow through the three

phases of the MMC, which increases the rating values of the MMC devices, the converter

power losses, and the SM capacitor voltage ripple. The circulating current is caused by

the inner voltage differences, and they are in the form of negative sequence with the

frequency as twice the fundamental wave. In the normal operation of the MMC, the

circulating current should be eliminated.

According to (4-2), the second harmonic circulating current can be described as

)3

22sin(

)3

22sin(

)2sin(

2_2

2_2

2_2

tIi

tIi

tIi

ofmcf

ofmbf

ofmaf

(4-6)

where I2fm is the peak value of the second harmonic circulating current. ωo is the

fundamental frequency. φ is the initial phase angle.

Based on Fig. 4.7, the inner dynamic of the MMC with the circulating current can be

defined as


60

f_c2

f_c2

_2

f_b2

f_b2

_2

f_a2

f_a2

_2

iRdt

diLu

iRdt

diLu

iRdt

diLu

sscf

ssbf

ssaf

(4-7)

where u2f_j is the inner voltage of phase j (j=a, b, c). The (4-7) can be transferred into the

dq reference frame with the transformation matrix below

)3

2sin()

3

2sin(sin

)3

2cos()

3

2cos(cos

3

2/

ooo

ooo

dqabcT (4-8)

qf

df

s

qf

df

so

so

qf

df

s

qf

df

i

iR

i

i

L

L

i

i

dt

dL

u

u

_2

_2

_2

_2

_2

_2

_2

_2

02

20

(4-9)

where u2f_d and u2f_q are the inner voltage in the dq reference frame, respectively. i2f_d and

i2f_q are the second harmonic circulating current in the dq reference frame, respectively.

θo=ωot.

A control structure is presented to eliminate the circulating current as shown in Fig.

4.10. According to (4-2), the difference current can be obtained by the summation of the

upper arm current iuj and the lower arm current ilj of phase j (j=a, b, c). And then, the

three-phase circulating current can be transformed to the dq reference frame as i2f_d and

i2f_q. In Fig. 4.10, the reference values are set as zero, and the simple PI controllers are

used here to eliminate the current i2f_d and i2f_q. Finally, the three reference values ua_2f,

ub_2f and uc_2f are generated. Here, Tdq/abc= Tabc/dq-1

.

++

iuj

ilj

1/2 Tabc/dq

2ωoLs

0+

-PI +

+

0 +-

PI +

i2f_d

-

ua_2f

ub_2f

uc_2f

i2f_q

2ωoLs

θo=2ωot

Tdq/abc

Fig. 4.10 Block diagram of the control for second circulating current elimination.


4.4.5 Onshore Converter Control

According to (4-5) and [99], the three-phase MMC can be decoupled into dq

reference frame as

q

o

gq

o

doq

o

omq

d

o

gd

o

qod

o

omd

eL

uL

iiL

R

dt

di

eL

uL

iiL

R

dt

di

11

11

(4-10)

)(2

3

)(2

3

mqdmpq

mqqmdd

ieieQ

ieieP

(4-11)

where ugd and ugq are the grid positive sequence voltage in the dq reference frame

respectively. ed, eq, and imd, imq are the inner voltage and grid current in the dq reference

frame respectively. Lo=Lf+Ls/2 and Ro=Rf+Rs/2 as shown in Fig. 4.7(a).

The onshore converter is used to maintain the HVDC link voltage close to the

reference by adjusting the active power transmitted to the onshore AC network to match

the power received from the offshore HVDC converter. The vector control technique has

been developed for the onshore current regulation. It is illustrated in Fig. 4.11, where the

subscripts d and q refer to the d, q-axis quantities. The MMC is controlled in a

synchronous rotating d, q-axis frame with the d-axis oriented along the grid voltage

vector position, which ensures the decoupling control of active and reactive powers into

the AC grid. The double control loops are used for the onshore converter. The grid

current is controlled in the inside loop. The outside loop is used to maintain the

transmission level DC voltage Vdc. The system reactive power can also be regulated with

the outside loop. One thing to be mentioned is that, the circulating current elimination

control signals ua_2f, ub_2f and uc_2f are inserted into the onshore converter control, which

can effectively suppress the circulating current in the three phases of the MMC.


62

Modular

Multilevel

Converter

Lf

ua

ub

uc

PWM

Modulatordq

abc

iga, igcigb,

uga, ugcugc,

-+udc

-+uqc

imd_ref

imd

imq_ref

imq

Vdc_ref+

+

-

-

+

+

-

-

udc = ugd + ωoLoimq

uqc = ugq - ωoLoimd

Q

Qref

Grid power

calculation dq

abc

ugd, ugq

imd, imq

PLLωo d/dt

Vdc

AC Grid

ua_2f ub_2f uc_2f

+

+

++

+

+

PI

PI

θo

PI

PI

Fig. 4.11. Block diagram of the control for the three-phase MMC.


4.5 Offshore Converter

4.5.1 Offshore Converter Configuration

The offshore converter is a key component in the DC-grid offshore wind farm, which

is a DC/DC converter to boost the collection level voltage to the transmission level

voltage. Fig. 4.12 shows the offshore converter. Owing to the advantages of small

harmonics, the three-phase MMC is used. The output of the DC/DC converter will take a

high voltage level. Hence, the diode rectifier is used at the output side. The three-phase

MMC at the input side converts the DC voltage into AC voltage. The frequency of the

AC voltage is higher than 50 Hz used in the normal AC system, which can significantly

reduce the size and the weight of the transformer. Afterwards, the AC voltage is stepped

up by the transformer and rectified into DC at the output side.

+

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

Co

MFT

Fig. 4.12. Block diagram of the offshore converter.

4.5.2 Offshore Converter Control

The modulation strategy and the voltage balancing control strategy for the three-

phase MMC in the offshore converter is the same to that in the onshore converter, which

is not repeated here.

The offshore converter is composed with POFC and NOFC, which are connected in

series. The NOFC is used to maintain the collection level voltage close to the specified

reference level, and the POFC is used to track the power of the NOFC.


64

The proposed offshore converter control is shown in Fig. 4.13. Fig. 4.13 (a) is used

for NOFC to keep the collection level voltage constant. A simple PI controller is used as

the DC-link voltage controller to generate the required modulation m. And then, the

PWM generator will produce the corresponding pulses to drive the MMC so as to ensure

that the collection level voltage VMV tracks the reference voltage VMV_ref. A power control

loop is used here to limit the NOFC power. If the NOFC power Poff2 is over the limit

value Poff2_lim of 1.1 p.u., the power control loop will limit the NOFC power.

Fig. 4.13(b) is used for the POFC to make its power Poff1 follow the NOFC power

Poff2, which is similar to Fig. 4.13(a). A PI controller is used as the power controller to

generate the required modulation ratio m. And then, the PWM generator will produce the

corresponding pulses to drive the MMC. Finally, the POFC power Poff1 follows the

NOFC power Poff2.

+-VMV_ref SMs

NOFC

VMV

-+

Poff2

Poff2_lim

+-

mc1

msin(ωt)

PWM

GeneratorPI

PI

(a)

+-Poff2 SMs

POFC

Poff1

msin(ωt)

PWM

GeneratorPI

(b)

Fig. 4.13. Block diagram of (a) The control for NOFC. (b) The control for POFC.


4.6 AC Grid Faults Ride Through

4.6.1 AC Grid Faults

Owing to the continuous increase in wind power penetration level, the operation of

existing utility networks may be affected, especially the stability of the power system.

The grid codes are now being revised to reflect new requirements for wind turbine

integration into the network. According to the grid code, at a short-circuit fault in the

external AC grid, wind turbines should keep connection to the grid and restore their

normal operation after the fault is cleared [103], [104]. The reason is that, when the wind

power penetration level is high, the protective disconnection of a large amount of wind

power will be an unacceptable consequence that may threaten the power system stability

[105], [106]. The wind turbine must thus be equipped with fault ride-through capability

required in the grid codes. The American Wind Energy Association (AWEA) has

proposed low-voltage ride-through requirements for the interconnection of large wind

generators [16]. The proposed voltage requirement are described by Fig. 4.14, which

shows the grid voltage drop area where the wind generators must remain connected to the

grid.

-1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2 40

0 1 3

Gri

ds

lin

e v

olt

age

(p.u

.)

Fault Incidence

Times (s)

0.625

Fig. 4.14. Low voltage ride-through requirement by AWEA.

The DC grid is shown in Fig. 4.6, where the power flow can be described as shown

in Fig. 4.15. Neglecting the power electronics loss, the relationship of the power in the

DC grid can be expressed as

Pw= Pk+ Pg (4-12)

Pg= Pc+ Pca1 (4-13)

Pc= Poff+ Pca2 (4-14)

Poff= Pon+ Pca3 (4-15)


66

where Pw is the captured power from the wind. Pk is the kinetic energy in the wind

turbine. Pg is the generator power. Pca1 is the capacitor power in the wind turbine’s power

converter. Pc is the output power of the wind turbine. Pca2 is the capacitor power in the

collection system. Poff is the offshore converter power, which is the summation of Poff1

and Poff2. Pca3 is the capacitor power in the HVDC transmission system. Pon is the

onshore converter power. The generator controlled by generator-side converter captures

the power Pg from the wind power Pw, the power Pk is stored as the kinetic energy. And

then, the grid-side converter transmits the power Pc from the generator to the collection

level, while power Pca1 is stored in the capacitor between the generator-side converter

and the grid-side converter. Afterwards, the collection level power is sent to the

transmission level as Poff by the offshore converter, except that some is stored in the

collection level capacitors as Pca2. Finally, the AC grid extracts the power Pon from the

onshore converter with some power stored in the transmission level capacitors as Pca3.

Wind

power

Kinetic

energy

Generator

side

converter

Pw

Pk

Capacitor

energy

Pg

Pca1

Grid side

converter

Capacitor

energy

Pc

Pca2

Offshore

converter

Capacitor

energy

Poff

Pca3

Onshore

converterPon

Wind turbine level Collection level Transmission level

Fig. 4.15. Power flow in the DC grid for offshore wind farms.

When a disturbance occurs in the external on shore AC grid, a voltage dip occurs at

the AC output terminal of the DC grid. The maximum active power that the DC grid can

send to the AC grid is decreased with the dip of the terminal voltage. At this time, the

output power of the onshore converter is quickly reduced. However, the input power of

the onshore converter is nearly not changed. Therefore, there is a power imbalance in the

transmission level under the grid disturbance, which results in the increase of the

transmission level voltage, and may damage the HVDC transmission system.

The low voltage ride-through of the DC grid as shown in Fig. 4.6 is studied here. The

configurations and the control strategies for the wind turbine, the offshore converter, and

the onshore converter in normal situation have been introduced in detailed before. The

response of the DC grid under AC grid disturbance is analyzed here, which mainly

focuses on the voltage dip situation at the point of common coupling (PCC). An effective

control strategy is presented to improve the faults ride-through capability of the DC grid,

which is significant to limit the DC-link voltage fluctuation in a small range, and avoid

the system damage. The presented control for faults ride-through mainly includes onshore

converter control, offshore converter control, and the wind turbine control, which will be

discussed below.


4.6.2 Offshore Converter Control under Faults

In order to improve the performance of the offshore converter under faults and avoid

overvoltage situation in the HVDC transmission system, the control for the offshore

converter is improved with the dotted box area as shown in Fig. 4.16, which is based on

Fig. 4.13. In Fig. 4.16, a voltage limit loop is inserted into the controller, which can

effectively limit the overvoltage in the HVDC transmission level under grid faults.

+-VMV_ref

VMV

-+

Poff1

Poff1_lim

+-

mc1

mPI

PI

SMs

NOFCsin(ωt)

PWM

Generator

+-

Vf2_lim

Vf2

mcompPI

+-

(a)

+-Poff2 SMs

POFC

Poff1

msin(ωt)

PWM

GeneratorPI

(b)

Fig. 4.16. Block diagram of (a) The control for NOFC under grid faults. (b) The control

for POFC under grid faults.

In Fig. 4.16, when the output voltage Vf2 of the NOFC, as shown in Fig. 4.6, is

increased and over the limit value Vf2_lim, which is set as 1.1 p.u. under grid faults, the

voltage limit loop starts to be active to produce a modulation ratio compensation

component mcomp, which is embedded into the voltage control loop to reduce the

modulation ratio. As a consequence, the NOFC power Poff2 transmitted to the

transmission level is effectively reduced. According to (4-15), although the AC grid

power Pon is reduced owing to the voltage dip at PCC, the offshore converter power Poff

is also fast reduced, which could effectively reduce the capacitor power Pca3 and limits

the increase of the capacitor voltage in the transmission level.


68

On the other hand, based on (4-14), the decrease of the power Poff sent to the

transmission level by the offshore converter could result in the increase of power Pca2.

Consequently, the overvoltage may appear in the collection level. The following wind

turbine control could effectively handle this problem.

4.6.3 Wind Turbine Control under Faults

The wind turbine I associated with its control is used in the DC grid, which is

composed with the grid-side converter control and the generator-side converter control.

Grid-Side Converter Control

As the aforementioned introduction, the control for the offshore converter results in

that more power is stored in the collection level, which could make the DC-link voltage

in the collection level increased. Here, an improved control is presented for the grid-side

converter in the wind turbine, which can effectively avoid the overvoltage in the

collection system.

According to the control for the grid-side converter in normal situation in Chapter 3,

the control for the grid-side converter is improved with the dotted box area as shown in

Fig. 4.17. The output voltage of the wind turbine Vwt is measured by the control system

and compared with the limit value Vwt_lim, which is set as 1.1 p.u. in this study. Once the

wind turbine output voltage Vwt is over the value Vwt_lim, the reference current is reduced

by the voltage limit loop, which indirectly results in the decrease of the output power Pc

of the wind turbine. Consequently, according to (4-14), the capacitor energy in collection

level could not be increase so much, and the voltage in the collection level is effectively

limited.

+-Vdc_ref

+-

iL

PWM

generator

sw1~sw8Gi(s) Gu(s)

Vdc

+-

Vwt_limGuc(s)

Vwt

+-

iL_comp

iL_ref D IFBTL

DC/DC

Converter

Fig. 4.17. Block diagram of the improved control for the grid-side converter.


Generator-Side Converter Control

According to (4-13), owing to the reduction of the wind turbine output power Pc by

the improved control for the grid-side converter, more power would be stored in the DC-

link capacitor in the wind turbine power converter, which may result in that the capacitor

voltage is increased in the wind turbine. Here, an improved control is presented for the

generator-side converter so as to prevent the overvoltage.

PMSG

isa, iscisb,

dq

abcisd

isq

PWM

Generatordq

abc

Rotor

Position

measure

θgd/dt

udcom = -Lsqωrisq

uqcom = Lsdωrisd - ωrψr

ua

ub

uc

udcom

uqcom

Ggi(s) id_ref=0

isd

+-

+ -

isq

Ggp(s) Pg_ref

+

-

Pg ++

++

ωr

C+

Vdc

Ggi(s) iq_ref

-

Ggc(s) iq_comp

+

Vdc_lim

Vdc

+-

Fig. 4.18. Block diagram of the improved control for the generator-side converter.

The generator-side converter control is improved with the dotted box area, as shown

in Fig. 4.18. When the DC-link voltage Vdc in the wind turbine is higher than the

reference value Vdc_lim as 1.1 p.u., the compensation is active in the power control loop,

which is used to reduce the active power Pg of the generator. According to (4-13), it can

be seen that with the decrease of the generator power Pg, the capacitor power in the wind

turbine is effectively limited. Hence, the overvoltage in the DC link of the wind turbine

converter is avoided. According to (4-12), it can be realized that more power is

transferred to kinetic energy because of the decrease of the generator power. As a

consequence, the kinetic energy is increased with the increase of the wind turbine speed.

However, the inertia constant for the MW-level wind turbine is big and within the range

of 2-9 s. As a consequence, the wind turbine speed is only increased a little during the

short fault period. On the other hand, if the wind turbine speed is over the rated value, the

pitch angle control system will be active to limit the wind turbine speed.


70

4.6.4 DC Grid Performance under Faults

The performance of the DC grid under AC grid faults is studied with the professional

tool PSCAD/EMTDC. A 400 MW DC grid for offshore wind farm is modeled as shown

in Fig. 4.6, where four aggregated variable speed wind turbines with 100 MW each

respectively are used in the wind turbine system. It is assumed that each aggregated

model has twenty of 5 MW wind turbines lumped together. The three-phase short-circuit

fault happened on one of the 110 kV double circuit transmission lines, and result in that

the voltage Vpcc at the PCC is dipped to 15% of the rated value. After 150 ms, the fault is

cleared from the power system and the voltage of PCC is recovered. The wind turbine

parameters, cable parameters, and the HVDC system parameters are shown in the

Appendix A, D, and E, respectively. The aforementioned control is applied for the DC

grid, and the DC grid performance is shown in Fig 4.19~Fig. 4.21.

Fig. 4.19(a) shows the wind speeds for the wind turbines in the wind farm. A grid

fault happened at 7 s causes AC grid voltage Vpcc dip to 15% of the rated value and

lasting for 150 ms as shown in Fig. 4.19(b).

Fig. 4.20 shows the performance of the DC grid only with the normal control and not

with the improved control under faults. In normal situation, these wind turbines in the

wind farm are controlled to capture the optimal power from the wind by the generator-

side converter associated with the pitch angle control system. The grid-side converter

keeps the DC-link capacitor voltage constant as 5.4kV in the wind turbine for the normal

operation of the generator-side converter. The collection level voltage is kept by the

offshore converter as 40 kV and the transmission level voltage is maintained as 150 kV

by the onshore converter before grid faults.

Once the grid fault happened at 7 s, and the AC grid voltage Vpcc dipped to 15% of

the rated value, the power sent into the grid is reduced in proportional to the decrease of

the AC grid voltage shown in Fig. 4.20(i). At this time, the onshore converter loses its

controllability, and could only send little power to the AC grid. However, the power

transferred from the offshore converter is nearly unchanged. Consequently, more energy

is stored on the transmission level, which results in the increase of the transmission level

voltage. It can be seen that the transmission level voltage is nearly increased more than

four times as shown in Fig. 4.20(h). Figs. 19(a)~(g) shows the performances of the wind

turbines and the collection system.

Fig. 4.21 shows the DC grid performance with the improved control under faults.

During the grid faults, if the transmission level voltage is measured to be over the limit

value as 1.1 p.u., the offshore converter starts to reduce its input power so as to limit the


increase of transmission level voltage. Fig. 4.21(h) shows that the transmission level

voltage, which is only increased to 1.2 p.u.. Because of the decrease of the input power

for the offshore converter, the imbalance appears in the collection level, which causes the

increase of the collection level voltage. With the improved control under faults, the grid-

side converter in the wind turbine reduces its output power as shown in Fig. 4.21(e) if its

output voltage is over the reference value as 1.1 p.u.. Hence, the collection level voltage

is effectively limited with the maximum value as 1.2 p.u. shown in Fig. 4.21(f). Also, the

DC-link voltage in the wind turbine converter is increased because of the decrease of its

output power. With the improved control under faults, the generator-side converter starts

to reduce its output power shown in Fig. 4.21(c) to keep the DC-link voltage in the wind

turbine if the DC-link voltage is over the set point as 1.1 p.u. as shown in Fig. 4.21(d).

Finally, the decrease of the generator power causes the increase of the kinetic energy. As

a consequence, the wind turbine speeds are increased, as shown in Fig. 4.21(a).

Nevertheless, owing to the action of pitch angle control system as shown in Fig. 4.21(b)

and the big inertia of the wind turbine, the wind turbine speed could not have a big

change during the short time period. As a consequence, the DC grid for the offshore wind

farm could have a good performance with the improved control under faults.

(a) (b)

Fig. 4.19. (a) Wind speed. (b) PCC voltage.

(a) (b)

(c) (d)


72

(e) (f)

(g) (h)

(i)

Fig. 4.20. Performance of the DC-grid offshore wind farm without the improved control.

(a) Wind turbine speed. (b) Pitch angle. (c) Generator power. (d) DC-link voltage in the

wind turbine. (e) Wind turbine output power. (f) Collection level voltage. (g) Offshore

converter power. (h) Transmission level voltage. (i) Grid power.

(a) (b)


(c) (d)

(e) (f)

(g) (h)

(i)

Fig. 4.21. Performance of the DC-grid offshore wind farm with the improved control. (a)

Wind turbine speed. (b) Pitch angle. (c) Generator power. (d) DC-link voltage in the wind

turbine. (e) Wind turbine output power. (f) Collection level voltage. (g) Offshore

converter power. (h) Transmission level voltage. (i) Grid power.

4.7 Summary

In this chapter, the HVDC transmission system in the DC grid is introduced. The

MMC-based VSC configuration associated with its control is presented for the onshore

converter. Owing to the special requirement for the offshore converter in the DC grid, a

MMC-based DC/DC converter is presented, and the corresponding control is introduced.

A medium frequency transformer is used in the offshore converter, which may reduce the

size and weight of the offshore converter. The ride-through control for the AC grid fault

is presented. Through the improvement of the offshore converter control and the wind

turbine control, the performance of the DC grid under AC grid faults can be effectively

improved.

Protection and Redundancy under HVDC System Faults

74

Chapter 5

Protection and Redundancy under

HVDC System Faults

5.1 Introduction

At present, the collection system in all existing offshore wind farms is an AC system.

Through the high voltage AC or DC transmission technology, the collected AC power is

transmitted to the onshore AC grid. As a consequence, the existing grid codes for wind

turbines and the investigation on low voltage ride through capabilities are mainly focused

on the AC systems.

As to the DC grid for offshore wind farm, a promising wind farm system in future,

where the DC collection system is used instead of the AC collection system [1-10], it also

requires the corresponding grid codes for its development. Unfortunately, there are rarely

grid codes for DC transmission and collection systems of the wind farm. Therefore, the

behaviors, protection, redundancy, and control and so on are needed to be studied for the

offshore wind farm with DC grid.

In the DC grid for the offshore wind farm, the fishing activities, anchors, ageing

phenomena and so on, would cause the cable faults with the possibility as approximate 1

fault/100 km/year [107], which may result in the HVDC transmission system fault and

may damage the DC grid. The repair for the damaged system in the offshore wind farm is

costly, and often takes considerable time, even several months, which results in that the

possible loss of income is enormous [108]. Normally, the overvoltage and overcurrent

situations may be caused in the HVDC transmission system because of the DC cable fault,

which is very harmful to the HVDC transmission system and may damage the onshore

75 Protection and Redundancy under HVDC System Faults

converter and the offshore converter and should be avoided. As a consequence, the

protection for the DC grid is very important to enhance its reliability and save the costs.

To date, a few protective devices have been developed such as AC CBs, DC CBs,

and DC switches and so on. The DC CB based on ETO thyristor has been introduced in

some literatures [26], [29]. Although the DC CB has fast switch speed in less than 10 us,

and can be used in series with capacitor to limit and interrupt the capacitor discharge

current during a fault, this kind of DC CB based on ETO may not be used in this high

voltage DC system because of its low voltage capacity. The references [27] and [28]

presents the new type of active DC CB based on standard SF6 AC circuit breakers with

auxiliary circuits, which normally needs tens of milliseconds to interrupt the DC circuit,

but this active DC CB may also not be fast enough to protect the transmission system

since the capacitor in the onshore or offshore converters discharges very fast under DC

faults.

In this chapter, the cables short-circuit fault of the HVDC transmission system is

studied in the DC grid for offshore wind farms. The dynamic performance of the DC grid

under faults is analyzed, and the corresponding protection is presented. In addition, the

protection design is also introduced, which can effectively protect the DC grid and avoid

the damage, and significantly enhances the reliability of the HVDC transmission system.

On the other hand, the redundancy of the HVDC system under DC cable faults is

presented. Through the corresponding operation of switchgears and the related control,

the DC grid can effectively ride-through the HVDC system fault. The DC grid for

offshore wind farms is modeled and simulated, and the results validate the feasibility of

the presented protection, redundancy, operation, and control.


76

5.2 DC Cable Faults

The DC grid for the offshore wind farm is shown in Fig. 4.6, which is composed

with the wind turbines, DC-grid collection system and HVDC transmission system and so

on. Fig. 5.1 shows the HVDC transmission system in detail, which consists of the PONC,

NONC, POFC, NOFC, and transmission cables. In case of breakdown of anyone

converter, the transmission system can still be operated with the other healthy converters.

As a consequence, this type configuration as shown in Fig. 5.1 can provide a high degree

of reliability for the HVDC transmission system. Although the MMC-based HVDC

system maybe evaluated to be attractive in the wind power industry, such as the Trans

Bay Cable project [67], the two-level and three-level voltage source converters have been

widely used for the HVDC transmission system. As a consequence, the two- and three-

level VSC are considered here for the onshore converter in the HVDC transmission

system.

if1

Vf1 Vn1

Sw1

Sw2

Sw3

Sw4

Sw5

Sw6

ABC

DC Cable

F

Positive offshore converter

（POFC）Positive onshore converter

(PONC)

icn1

Cn1++

Cf1Rf Lf

Negative offshore converter

(NOFC)

Negative onshore converter

(NONC)

+Cf2

Vf2 Vn2

Cn2+

in1

icf1

ifc1 ivsc1

CB1

CB2Rf Lf

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

Fig. 5.1. HVDC transmission system with positive pole cable-to-ground fault.

In this chapter, the positive pole cable-to-ground fault on the HVDC transmission is

considered and studied as shown in Fig. 5.1. The possible impacts of the fault on the

onshore converter and offshore converter are analyzed and discussed. Furthermore, the

corresponding protection is presented and designed. The negative pole cable-to-ground

fault can also be treated with the same method.


5.3 HVDC System Performance under Faults

5.3.1 Dynamic Performance of Onshore Converter

Fig. 5.2 shows the equivalent circuit of the onshore converter under faults. During

the fault, the IGBTs are all switched off. The ea, eb and ec are equivalent transformer

voltage. Lf and Rf are the filter inductance and resistance. Leq and Req are the equivalent

inductance and resistance. The cable-to-ground fault happened at the point F. ln is the

cable length between the PONC and the fault point F.

+

ia1

ib1

ic1

Rf Lf

in1

ida1

Da1

idb1 idc1

Db1 Dc1

Da2 Db2 Dc2

ida2 idb2 idc2

ivsc1

ln

F

ab

c

icn1

cable

Leq ea

eb

ec

O

Req

Cn1 Vn1

Fig. 5.2. Equivalent circuit for the onshore converter under faults.

Once the cable-to-ground fault happens as shown in Fig. 5.2, the cable current at the

receiving end, -in1, reduces to 0 firstly, and then reverses the current direction while the

capacitors Cn1 at the terminal starts to discharge into the fault point F, which may result

in a high short circuit current in the system, even that the PONC IGBTs are blocked for

protection purpose during the fault.

The capacitor Cn1 will be bypassed by the diodes when the capacitor voltage Vn1

collapses and is reduced to 0. Afterwards, the cable inductance starts to discharge and the

cable current circulates in the freewheel diodes with an initial value. Each phase-leg

carries a third of the cable current. As a consequence, a huge current suddenly appears in

the diodes when the capacitor is bypassed by the diodes, which may damage the diodes.

On the other hand, the short circuit fault causes a transient performance at the AC

side, where the AC current ia1, ib1, and ic1 are fed into the DC-link via diodes. After the

capacitor voltage collapses within a very short time and the capacitor is bypassed by the

diodes, the AC side is short circuit at the points a, b, and c. Suppose that the AC grid

phase a voltage after the short circuit occurs is expressed as


78

ea = Um sin(ωt+α) (5-1)

where Um is the amplitude of transformer phase voltage. ω is the synchronous angular

frequency. α is the phase angle. The phase a current under the AC grid three-phase short

circuit can be obtained as (5-2) [26].

ia1(t) = Im sin(ωt+α-φ) + [Im0 sin(α-φ0) - Im sin(α-φ)]e-t/τ

(5-2)

with Im2=Um

2/(R

2+ω

2L

2), φ=arctan(ωL/R), L=Lf+Leq, R=Rf+Req, τ=L/R, Im0 and φ0 are the

grid current amplitude and phase angle at the beginning of the AC grid three-phase short

circuit. The phase b current ib1(t) and the phase c current ic1(t) can be obtained with the

same method, which are not repeated here.

According to the parameters in the Appendix E, suppose the initial values of Im0 is 0,

the most serious transient current ia1 at the AC side with the maximum value of

approximate 11.1 kA is plotted as shown in Fig. 5.3 under the condition that the grid

voltage phase angle is zero at the short circuit initiation [26]. Although the AC CB is

equipped at the AC side as shown in Fig. 5.1, it may not be fast enough to avoid the large

current because approximate tens of milliseconds is required for the AC CB to interrupt

the circuit [16].

Fig. 5.3. The most serious transient current ia1 at AC grid under faults.

The performance of the onshore converter under faults is analyzed with Fig. 4.6. The

positive pole cable-to-ground fault is considered as shown in Fig. 5.1. The fault resistance

is very small, and considered as zero in this study. The different fault distances ln

including 0.1 km, 1 km and 10 km are conducted respectively. Suppose the positive

onshore converter is initially operated with the rated power Prated as 200 MW, the cable

initial current is the rated value as In10 =Prated÷Vn1 =200MW÷150kV=1.33kA.

The simulation results are shown in Fig. 5.4~Fig. 5.5. Under faults, the DC-link

voltage Vn1 collapses fast in less than 1 ms. Along with the reduction of the fault distance

ln, the voltage Vn1 collapses faster, and the peak value of the cable current would be


higher. The maximum cable current nearly reaches 68 kA when ln=0.1 km. When the

voltage Vn1 is reduced to 0, the capacitor is bypassed by the diodes. As a consequence,

the huge cable current abruptly flows through the freewheel diodes, which may damage

the diodes. The most serious AC grid transient current ia1 under ln=0.1 km is shown in

Fig. 5.4(d) with the maximum value of approximate 11 kA, which is quite close to the

value as shown in Fig. 5.3.

(a) (b)

(c) (d)

Fig. 5.4. Onshore converter performance under the fault distances ln as 0.1 km. (a)

Voltage Vn1. (b) Cable current in1. (d) Diode current. (g) Grid current.

(a) (b)

(c) (d)

Fig. 5.5. Onshore converter performance under the fault distances ln as 1 km. (a) Voltage

Vn1. (b) Cable current in1. (d) Diode current. (g) Grid current.


80

(a) (b)

(c) (d)

Fig. 5.6. Onshore converter performance under the fault distances ln as 10 km. (a)

Voltage Vn1. (b) Cable current in1. (d) Diode current. (g) Grid current.

5.3.2 Dynamic Performance of Offshore Converter

Fig. 5.7 shows the equivalent circuit of the offshore converter, where the cable-to-

ground fault happened at the point F. The lf is the distance between the POFC and the

fault point F. During the fault, the IGBTs of the POFC are blocked if the cable current if1

is over the IGBT protection value, and the wind farm power will be blocked by the POFC

and cannot be sent to the grid with the positive-pole cable. As a consequence, only the

diode rectifier at the output side of the offshore converter reacts to the cable-to-ground

fault under faults, and the equivalent circuit of the offshore converter under faults would

be a diode rectifier.

During the cable-to-ground fault, the offshore converter performance only contains

capacitor discharge phenomenon and diode freewheel phenomenon. Once the cable-to-

ground fault happens, the capacitor at the offshore converter discharges very fast. When

the capacitor voltage Vf1 is reduced to 0 and the capacitor is bypassed by the diodes, the

cable current starts to circulate in the freewheel diodes. Owing to three diode phase-legs

in the offshore converter, each phase-leg takes one third of the cable current.


if1

Vf1

D3 D5

D4 D6

+Cf1

icf1

ifc1

lf

idf3 idf5

idf4 idf6

Fcable

D1

D2

idf1

idf2

Fig. 5.7. Equivalent circuit for the offshore converter under faults.

Fig. 5.8~Fig. 5.10 shows the offshore converter performance under faults, where the

different fault distances lf including 0.1 km, 1 km and 10 km are conducted respectively.

Suppose the POFC is initially operated with the rated power Prated as 200 MW, the

capacitor initial voltage is approximately Vf10=Vn1+If10×Rc×Lcable=150kV+1.33kA×0.0151

Ω/km×100km =152kV, where Rc is the cable resistor (Ω/km), and Lcable is the cable

length (km). The cable initial current If10 is the rated value as 1.33kA.

According to the simulation results, the DC voltage Vf1 collapses in less than 1 ms

under faults. Along with the increase of the fault distance lf, the DC voltage collapses

slower and the maximum value of the capacitor discharge current icf1 and cable current if1

becomes smaller. After the voltage Vf1 is reduced to 0 and the capacitor Cf1 is bypassed

by diodes, the cable current starts to circulate in the diodes with a big initial value. In this

situation, each diode phase-leg takes one third of the cable current, which is still very

huge and may damage the diodes.

(a) (b)

(c) (d)


82

Fig. 5.8. Offshore converter performance under the fault distances lf as 0.1 km. (a)

Voltage Vf1. (b) Capacitor discharge current icf1. (c) Cable current if1. (d) Diode current.

(a) (b)

(c) (d)

Fig. 5.9. Offshore converter performance under the fault distances lf as 1 km. (a) Voltage

Vf1. (b) Capacitor discharge current icf1. (c) Cable current if1. (d) Diode current.

(a) (b)

(c) (d)

Fig. 5.10. Offshore converter performance under the fault distances lf as 10 km. (a)



5.4 Protection for HVDC System

5.4.1 Protective Inductor for Onshore Converter

Normally, the IGBTs of the onshore converter will be switched off under faults, and

only diodes in the onshore converter may carry current. In this situation, there will be two

situations for each phase-leg in Fig. 5.2. One situation is that both of the diodes in the

phase-leg are conducted, the other situation is that only one diode in the phase-leg is

conducted.

1) Both diodes in one leg in conduction

In Fig. 5.2, if the two diodes in phase x (x∈ (a, b, c)) are both conducted, the diodes

current in this phase-leg can be expressed as

2/)(2/))()(()(

2/)(2/))()(()(

1212

1211

titititi

titititi

xdxdxdx

xdxdxdx (5-3)

If both of the two diodes in the phase-leg are conducted, there will be the following

relationship

(idx1(t) + idx2(t))/2 > |ix1(t)|/2 (5-4)

The maximum diode current id1_max in this situation can be presented as

id1_max = Max[(ida1(t)+ida2(t))/2+|ia1(t)|/2, (idb1(t)+idb2(t))/2+| ib1(t)|/2,

(idc1(t)+idc2(t))/2+|ic1(t)|/2] (5-5)

If the diodes in any two different phases α and β (α, β∈ (a, b, c)) are all conducted in

Fig. 5.2, there will be the relationship as

(idα1(t) + idα2(t))/2 = (idβ1(t) + idβ2(t))/2 (5-6)

Based on (5-3) and (5-6), the (idx1(t) + idx2(t))/2 possibly gets its maximum value

when the diodes in the three phase-legs are all conducted, which is a third of the cable

current and can be expressed as

(idx1(t) + idx2(t))/2 ≤ in1(t)/3 (5-7)

Substituting (5-7) into (5-5), the possible maximum diode current in this situation

can be obtained as

id1_max = Max[in1(t)/3+Max[|ia1(t)|, |ib1(t)|, |ic1(t)|]/2]


84

≤ Max[in1(t)]/3+Max[|ia1(t)|, |ib1(t)|, |ic1(t)|]/2 (5-8)

The Max[in1(t)] may be reached when the capacitor voltage Vn1 is reduced to zero

under faults, which can be expressed as in1m and discussed later. The Max[|ia1(t)|, |ib1(t)|,

|ic1(t)|] can be derived from (5-2) and Fig. 5.3 under the most serious transient situation,

which can be presented as

Max[|ia1(t)|, |ib1(t)|, |ic1(t)|] = Im [1+e-1/(2fτ)

] (5-9)

where f is AC grid frequency. Substituting (5-9) into (5-8), the possible maximum diode

current in this situation can be obtained as

id1_max ≤ in1m/3+ Im [1+e-1/(2fτ)

]/2 (5-10)

2) One of diodes in one leg in conduction

If (5-4) is not true and (idx1(t) + idx2(t))/2=|ix1(t)|/2, one of the two diodes in the phase

x would be blocked, and the other one takes the current |ix1(t)| in Fig. 5.2. As a

consequence, the possible maximum diode current id2_max in this situation can be obtained

when the most serious transient situation occurs, and id2_max can be expressed as

id2_max = Max[|ia1(t)|, |ib1(t)|, |ic1(t)|] = Im [1+e-1/(2fτ)

] (5-11)

To reduce the diode current and protect converters under faults, it should be

id1_max ≤ id2_max (5-12)

The id2_max is considered as the possible maximum fault current under faults.

In order to protect the system under faults, a protective inductor, Lpn, is designed as

shown in Fig. 5. 11. The protective inductor is installed at the terminal of the onshore

converter to prevent overcurrent and makes (5-12) satisfied under faults, therefore, the

diodes in the converters can be protected.

in1

Vn1

Cn1+

Lpnln

F cable

icn1

Fig. 5.11. Protection design for the onshore converter.


Suppose that the initial capacitor current Icn10 is 0 in Fig. 5.11, if the fault distance ln

is 0, the possible maximum cable current in1m can be obtained as (5-13) when the

capacitor voltage is reduced to zero.

pnnnmn LCVi 1101 (5-13)

To get the relationship between the current in1m and the inductor Lpn, the initial

capacitor voltage Vn10 is set as 165 kV with a margin of 1.1 for the 150 kV transmission

system because of the possible voltage variation. Along with the increase of the

inductance value Lpn, the possible maximum cable current in1m is reduced as shown in Fig.

5.12. As a consequence, the diode freewheel current is also decreased.

Fig. 5.12. The possible maximum values for capacitor Cn1 discharge current icn1 under

different inductance Lpn.

In order to protect the diode, limit the cable current and satisfy (5-12), the protective

inductor Lpn can be designed. According to (5-13), a 6.5 mH inductor Lpn is selected.

With the protective inductor, (5-12) is satisfied, and the maximum capacitor discharge

current is limited as 12.2 kA as shown in Fig. 5.12. According to (5-2) and Fig. 5.3, the

possible maximum diode current is approximate 11.1 kA. As a consequence, the capacity

of the diodes in the onshore converters can be designed as 12.2 kA with a margin of 1.1.

5.4.2 Dynamic Performance of Onshore Converter under Protection

The DC grid for the offshore wind farms has been modeled with the professional tool

PSCAD/EMTDC as shown in Fig. 4.6. The positive pole cable-to-ground fault is

considered in the simulation as shown in Fig. 5.1. The fault resistance is very small, and

considered as zero in this study.

With the proposed protection, a 6.5 mH protective inductor is equipped in the

onshore side, and Fig. 5.13~Fig. 5.16 shows the simulation results. In Fig. 5.1, the

positive pole cable-to-ground fault happened at 0.4 s, and the different fault distances are

studied including 0 km, 0.1 km, 1 km and 10 km.


86

With the protective inductor, the voltage Vn1 collapses slowly, and the peak value of

the cable current in1 is limited in a small range, which effectively reduce the diode current

and protect the diodes in the onshore converter. The AC currents under different fault

distances are similar.

(a) (b)

(c) (d)


Voltage Vn1. (b) Cable current in1. (c) Diode current. (h) AC grid current.

(a) (b)

(c) (d)

Fig. 5.14. Onshore converter performance under the fault distances ln as 0.1 km. (a)



(a) (b)

(c) (d)



(a) (b)

(c) (d)



The current components under the fault distance ln = 0 km is shown in Fig. 5.13(d)

and Fig. 5.17(a). There is the relationship -ib1(t)/2=Max[|ia1(t)|, |ib1(t)|, |ic1(t)|]/2>in1(t)/3

between 0.4057 s and 0.408 s. During this short time, 0.4057 s to 0.408 s, the D3 is

blocked and the D4 takes the maximum diode current |ib1(t)|. From 0.408 s to 0.4145 s,

there is ia1(t)/2=Max[|ia1(t)|, |ib1(t)|, |ic1(t)|]/2>in1(t)/3, which makes the diode D2 blocked

and the diode D1 flow through the maximum diode current ia1. The ia1/2 appears as the

maximum value of 5.5 kA at 0.411 s as shown in Fig. 5.18(a), and the maximum diode

current of 11 kA is caused, which could be tolerated by the selected onshore diodes

before. Figs. 5.17(b)~(d) shows the diode current in a short time under faults, when the


88

diodes in the three phase-legs are all conducted. In Figs. 5.17(b)~(d), the simulation

results are the same as the calculation results based on (5-3) and (5-6).

(a) (b)

(c) (d)

Fig. 5.17. (a) Current components under ln as 0 km. (b) Diode current ida1 and ida2 under

ln as 0 km. (c) Diode current idb1 and idb2 under ln as 0 km. (d) Diode current idc1 and idc2

under ln as 0 km.

5.4.3 Protection Inductor for Offshore Converter

Similar to the onshore converter, an inductor Lpf is designed and installed at the

terminal of the offshore converter as shown in Fig. 5.19, which is used to prevent

overcurrent and protect the diodes in the offshore converters.

if1

Vf1

D3 D5

D4 D6

+Cf1

icf1

ifc1

F

lf

cable

Lpf

D1

D2

Fig. 5.18. Protection design for the offshore converter.


Suppose that the fault distance lf is 0, the current icf1 as shown in Fig. 5.18 can be

expressed as

)1

sin()(110

101

1

2

10

12

101

f

fp

f

f

fpf

f

pf

f

fcfC

L

V

Itgt

CLI

L

CVti

(5-14)

The POFC is supposed to be operated with the rated power of 200 MW, the cable

initial current If10 is the rated value as 1.33 kA. Because of the voltage variation in the

transmission system, the capacitor initial voltage Vf10 is set with 1.1 margin as

1.1×(Vn1+Prated÷Vn1×Rc×Lcable)=167.2 kV. According to (5-14), Fig. 5.19 shows the

possible maximum capacitor discharge current values under different protective

inductance. In Fig. 5.19, the bigger of the selected protective inductance value, the

smaller of the possible maximum capacitor discharge current, and the diode freewheel

current can be limited in a small range.

Fig. 5.19 The possible maximum value for capacitor Cf1 discharge current icf1 under

different inductance Lpf.

In order to limit the capacitor discharge current and diode current, the offshore

protective inductor Lpf can be designed. According to (5-14), the Lpf is selected as 6.5 mH,

which can limit the maximum capacitor discharge current approximately to 12.4 kA as

shown in Fig. 5.19, and the possible maximum diode current is approximately 4.1 kA. As

a consequence, the capacity for the offshore diodes can be designed as 4.5 kA with a

margin of 1.1.

5.4.4 Dynamic Performance of Offshore Converter under Protection

Fig. 5.20~Fig. 5.23 show the offshore converter performance, where the 6.5 mH

protective inductor is equipped at the offshore converter. The positive pole cable-to-

ground fault happened at 0.4 s. The different fault distances such as 0 km, 0.1 km, 1 km

and 10 km are conducted.

With the protective inductor, the maximum capacitor discharge current icf1 and cable


90

current if1 is effectively limited. The maximum cable current is approximately 11.2 kA

under lf = 0 km as shown in Fig. 5.20(c). As a consequence, the diode currents under

different fault distances are also decreased, which is within the offshore diode capacity

selected before. Therefore, the offshore converter can be protected.

(a) (b)

(c) (d)



(a) (b)

(c) (d)

Fig. 5.21. Offshore converter performance under the fault distances lf as 0.1 km. (a)



(a) (b)

(c) (d)



(a) (b)

(c) (d)




92

5.5 Redundancy of HVDC System

In order to enhance the system reliability, the redundancy of the HVDC transmission

system under DC cable faults is an important issue, which can be realized by operating

the switches (DC switches S1-S12 and the AC circuit breakers CB1 and CB2), the

chopper resistors as shown in Fig. 5.24, and the corresponding control. In the normal

situation, the DC switches S1-S12, AC circuit breakers CB1 and CB2 are all closed so as

to ensure the normal operation of the system.

if1

Vf1 Vn1

Sw1

Sw2

Sw3

Sw4

Sw5

Sw6

ABC

DC Cable

F

Positive offshore converter

（POFC）Positive onshore converter

(PONC)

icn1

Cn1++

Cf1Rf Lf

Negative offshore converter

(NOFC)

Negative onshore converter

(NONC)

+Cf2

Vf2 Vn2

Cn2+

in1

icf1

ifc1 ivsc1

CB1

CB2Rf Lf

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

smooth inductor

S12

ic1

ic2

Scr1

Rcr1

Scr2

Rcr2

Fig. 5.24. Block diagram of the HVDC transmission system with detailed switchgears in

DC grid.

5.5.1 Onshore Converter Operation

Fig. 5.25 shows the operations for the onshore station. When the cable current in1 is

increased under faults, all the IGBTs of the PONC may be blocked for protection purpose

at tn1. At the same time, the CB1 is opened, which can interrupt the AC current after a

short interval at tn2 [72]. As a consequence, the in1 is reduced to 0 at tn3. And then, the S9

and S10 are opened to isolate the faulted cable.


PONC

CB1

S9

S10

0

on

off

close

open

tn1 tn3 t

-in1

close

open

close

open

tn2IGBT

protection

value

Fig. 5.25. DC transmission link fault current (onshore converter side) and components’

operation sequence of the onshore station.

5.5.2 Offshore Converter Operation

The offshore station operations are shown in Fig. 5.26. Once the cable current if1 is

measured over IGBT protection value under faults, all the IGBTs of the POFC are

blocked at tf1, which prevents the corresponding wind farm power from flowing into the

DC transmission link. When the if1 is reduced to 0 at tf2, the switches S5 and S6 are

opened to isolate the faulted positive pole cable. On the other hand, the POFC will be

isolated by the switches S1 and S2 when the current ic1 is reduced to 0.

POFC

S5

0

IGBT

protection

value

S6

on

off

close

open

if1

tf1 tf2 t

close

open

Fig. 5.26. DC transmission link fault current (onshore converter side) and components’

operation sequence of the onshore station.


94

5.6 Fault Ride-Through Control for HVDC System

The DC cable short-circuit fault is one of the most serious disturbances of the HVDC

transmission system, which may paralyze the DC grid. Therefore, the DC grid must have

the capability of the fault ride-through under the DC cable fault. To date, only a few

literatures report the DC cable faults in the DC grid. The basic control and design of the

DC grid for offshore wind farms are presented in [10], where the different DC wind farm

topologies are compared and the corresponding control is introduced. The handshaking

method is proposed in [30], which can locate and isolate the faulted DC line and restore

the DC system without telecommunication. The protection for the low voltage DC system

is discussed in [29], where the modern voltage-source converters are considered as fast-

acting current-limiting circuit breakers. The fault characteristic for the DC grid is

analyzed and some possible protection methods are given in [26]. A DC overvoltage

control during loss of converter in the multi-terminal VSC-HVDC system is presented in

[31]. However, the fault ride-through issue in the DC grid for offshore wind farms under

a DC transmission cable fault has not been mentioned in relevant grid codes and

literatures.

The fault ride-through control strategy in the DC grid for offshore wind farms is

considered here, which contains the onshore converter control, offshore converter control,

chopper resistor control, and the wind turbine control. The proposed fault ride-through

control can ensure the normal operation of the DC grid and send as much as possible

power to the grid under DC cable faults.

5.6.1 Onshore Converter Control

The onshore converters are used to keep the transmission level voltage constant,

whose control has been introduced in Chapter 4 and is not described in detail here. In

normal situation, the DC-link voltage Vn1 and Vn2 are regulated by the PONC and NONC

to be constant, respectively. The reactive power is controlled to be zero which means a

unity power factor. During the positive-pole cable fault, the NONC still keeps the DC-

link voltage Vn2 constant, while all the IGBTs of the PONC are blocked from tn1 as shown

in Fig. 5.25.

5.6.2 Offshore Converter Control

The NOFC is used to keep the collection level voltage VMV constant, and the POFC

would track the NOFC power, whose control has been presented in Chapter 4 in detail.


When the positive-pole cable-to-ground fault happened, all the IGBTs of the POFC are

blocked at tf1 as shown in Fig. 5.26, so as to prevent the wind farm power from flowing to

the faulted cable. As a consequence, only the NOFC is operated to keep the HVDC

system operation.

5.6.3 Chopper Resistor Control

If the IGBTs of the POFC are switched off at tf1 as shown in Fig. 5.26 under faults,

the wind farm power will be sent to the grid only with the NOFC. In this situation, when

the wind farm power is less than the capacity of the NOFC, the collection level voltage

VMV can be kept constant without overvoltage when the fault happened. On the contrary,

if the wind farm power is more than the capacity of the NOFC, the collection level

voltage VMV cannot be controlled. Here, the voltage VMV may be increased under faults,

and the overvoltage may be caused in the collection level when the fault happened.

The chopper resistor is equipped at the NOFC as shown in Fig. 5.24 to avoid the

overvoltage at the collection system. The control for the chopper resistor system is shown

in Fig. 5.27. If the VMV is out of control and increased over the limit value VMV_lim, set as

1.1 p.u., the chopper resistor system will cost some power and ensure the voltage VMV no

more than the VMV_lim.

+-

VMV_lim

VMV

PI0

1

D PWMGenerator

Scr

Fig. 5.27. Control for the chopper resistor.

5.6.4 Wind Turbine Control

Wind turbine control is an important issue for the fault ride-through. The grid-side

converter of the wind turbine is used to keep the DC-link voltage VLV constant, which has

been introduced in Chapter 3. The generator-side converter control under faults is shown

in Fig. 5.28(a), which is based on the dynamic model of the PMSG in the synchronous

rotating frame, with the d-axis being aligned with the rotor flux. The reactive power

reference is set to zero to perform unity power factor operation.


96

PMSG

id

iq

Rotor

Position

measure

θg

d/dt

PIid_ref=0

+-

+ -PI

Pg_ref-

Pg

ωr

PI+

Vdg

Vqg

MPPTPopt

0

1Pg_lim

Pup_lim

Power Selection Signal

SSP

0

(a)

Negative

offshore converter

WTs

WTs

Pup_lim

+-

1

Pnc

PI

0

1

Negative pole cable

(b)

Fig. 5.28. (b) Control for the generator. (c) Centralized power control for the offshore

wind farm.

Before tf1 in Fig. 5.26, the generator power control selection signal SSP is switched

to 0 as shown in Fig. 5.28(a), where the wind turbine captures its optimal power Popt

based on the maximum power point tracking (MPPT) method [36].

From tf1 in Fig. 5.26, the selection signal SSP is switched to 1. A limiter is inserted

into the generator controller as shown in Fig. 5.28(a). The assignment of the upper limit

Pup_lim is accomplished by a centralized control as shown in Fig. 5.28(b), where the

negative pole cable power is Pnc p.u.. If Pnc is less then 1 p.u., the centralized power

controller will increase Pup_lim to increase the wind farm power. In contrast, if Pnc is more

then 1 p.u., the centralized power controller will reduce Pup_lim to reduce the wind farm

power. Through the centralized power control, the power Pnc can be sent to the onshore

as much as possible within the cable’s capacity and the converter’s capacity. The offshore


wind farm performance may be described with the following two situations.

1) Pnc = 1 under condition ∑Popt_i ≥ 1

When the summation of each wind turbine optimal power Popt_i in the wind farm is

more than or equal to the negative pole cable’s capacity of 1 p.u., the centralized

controller will regulate Pup_lim to limit the power reference for the wind turbines, so as to

make the Pnc equal to the cable’s capacity.

As to some wind turbine, if the Pup_lim is more than its Popt_i, the wind turbine is

operated to track the optimal power with Pg = Popt_i < Pup_lim. If the Pup_lim is less than or

equal to its Popt_i, the Pg will follow Pup_lim with Pg = Pup_lim ≤ Popt_i.

2) Pnc = ∑Popt_i under condition ∑Popt_i < 1

If the summation ∑Popt_i of each wind turbine optimal power is less than 1, each

wind turbine would be controlled to follow its optimal power Popt_i by the centralized

power controller. Hence, the power Pnc is equal to ∑Popt_i.

5.7 System Performance under HVDC System Faults

The DC grid for offshore wind farms, as shown in Fig. 4.6, has been modeled using

the simulation tool PSCAD/EMTDC. The positive pole cable-to-ground fault occurs at 10

s, at 10 km away from the offshore station. The fault resistance is small and considered as

zero. The wind farm power is approximately 0.52 p.u. at the beginning of the fault. The

presented redundancy and the proposed control strategy are applied to the DC grid and

tested by simulation results.

Fig. 5.29 shows the simulation results. The variable wind speeds as shown in Fig.

5.29(a) are used to evaluate the performance of the DC grid. Owing to the fault at 10 s,

the offshore converter voltage Vf1 and the onshore converter voltage Vn1 are reduced to

zero as shown in Figs. 5.29(g) and (i), and the huge cable current if1 and in1 are caused.

During the faults, the IGBTs at the POFC are blocked, which results in that the cable

current if1 and in1 are reduced to zero as shown in Figs. 5. 29(h) and (j). And then the fault

cable is isolated by the switches S5 and S9. As a consequence, the POFC power and

PONC power are reduced to zero under faults as shown in Figs. 5.29(k) and (l).

When the fault happened at 10 s, a communication delay about 20 ms is set for the

wind turbines to get the signal to be operated with the proposed control under faults. At

this time, the wind farm power may be more than the NOFC capacity. Hence, the more

energy will be stored at the collection, which would increase the collection level voltage.

Based on the chopper resistor control before, if the collection voltage is over 1.1 p.u., the


98

chopper resistor will work to limit the collection level voltage as shown in Fig. 5.29(f).

After the wind turbines are controlled with the proposed control as shown in Fig.

5.28, the wind turbines power would be reduced to decrease the wind farm power as

shown in Fig. 5.29(d). The reduction of the wind turbines power results in that the wind

turbines speeds are increased as shown in Fig. 5.29(b). Because the wind turbine speeds

are not over the rated value, the pitch angles are still zero as shown in Fig. 5.29(c).

In the proposed wind turbine control under faults as shown in Fig. 5.28, the upper

limit Pup_lim is regulated by the central controller and sent to each wind turbine in the

offshore wind farm, so that the wind farm can produce the power as much as possible but

no more than the cable and converter’s capacity. Consequently, the Poff2 is quickly

regulated to 0.5 p.u. as shown in Fig. 5.29(k).

In Fig. 5.29(d), between 10 s and 16.1 s, the Popt1 is less than Pup_lim, the Popt2, Popt3,

and Popt4 are more than Pup_lim, which makes WT 1 follow its optimal power and WTs

2~3 follow the Pup_lim so as to keep the Poff as 0.5 p.u.. From 16.1 s to 17 s, Popt1 ~ Popt4

are all more than Pup_lim, and all the wind turbines follow the Pup_lim to make the Poff as 0.5

p.u.. From 17 s, the Popt1 is less than Pup_lim. Here, the WT 1 tracks the Popt1, and the

Pup_lim is increased to make the other three wind turbines produce more power to keep the

Poff as 0.5 p. u.. After 19.4 s, Popt2 is also less than Pup_lim, and the WT 2 follows the Popt3.

In order to produce more power, the Pup_lim is re-set by the central controller. Hence,

more power is produced by the WT 3 and WT 4. From 20.3 s, the Popt3 is less than Pup_lim

as well. Only WT 4 is left to follow the increased Pup_lim so as to produce more power. At

25.9 s, when the four wind turbines’ optimal power are all less than Pup_lim, all the wind

turbines follow their optimal power, the offshore wind farm power production becomes

less than 0.5 p.u. as shown in Fig. 5.29(k). The onshore converters power is shown in Fig.

5.29(l), where the negative onshore converter receives all the wind farm power via the

negative pole cable.

(a) (b)


(c) (d)

(e) (f)

(g) (h)

(i) (j)

(k) (l)

Fig. 5.29. (a) Wind speed. (b) Wind turbine speed. (c) Pitch angle. (d) Generator power

Pg1, Pg2, Pg3, Pg4. (e) Voltage VLV1, VLV2, VLV3 and VLV4 in the wind turbines. (f) Collection

level voltage VMV. (g) Voltage Vf1 and Vf2. (h) Cable current if1 and if2. (i) Voltage Vn1 and

Vn2. (j) Cable current in1 and in2. (k) Offshore converters power Poff1 and Poff2. (l) Onshore

converters power Pon1 and Pon2.


100

Fig. 5.30 shows the other simulation results with the high wind speeds, as shown in

Fig. 5.30(a). The DC-link voltage Vf1 and Vn1, the cable current if1 and in1, and the power

Poff1 and Pon1 are reduced to zero under faults as shown in Figs. 5.30(g)~(i). Owing to the

optimal powers of the WT 1~4 are more than the upper limit Pup_lim, the WT 1~4 are

operated with the Pup_lim, as shown in Fig. 5.30(d), which ensures the wind farm power as

much as possible, as shown in Fig. 5.30(k). The wind turbine speeds are suddenly

increased when the fault happened as shown in Fig. 5.30(b) because of the reduction of

the wind turbine power. The action of the pitch angle control, as shown in Fig. 5.30(c),

limits the wind turbine speed. The DC-link voltage in the wind turbines are shown in Fig.

5.30(e). There is a fluctuation at the collection level voltage under faults as shown in Fig.

5.30(f).

(a) (b)

(c) (d)

(e) (f)

(g) (h)


(i) (j)

(k) (l)

Fig. 5.30. (a) Wind speed. (b) Wind turbine speed. (c) Pitch angle. (d) Generator power

Pg1, Pg2, Pg3, Pg4. (e) Voltage VLV1, VLV2, VLV3 and VLV4 in the wind turbines. (f) Collection

level voltage VMV. (g) Voltage Vf1 and Vf2. (h) Cable current if1 and if2. (i) Voltage Vn1 and

Vn2. (j) Cable current in1 and in2. (k) Offshore converters power Poff1 and Poff2. (l) Onshore

converters power Pon1 and Pon2.

5.8 Summary

In this chapter, the fault analysis and protection design for the HVDC transmission

system under DC cable faults are presented. The transient characteristic of the HVDC

system under cable-to-ground fault is analyzed in detail. A huge current may be caused at

the onshore and offshore converters during the fault, which may damage the diodes in the

converters. The protective inductor is presented and designed for the onshore and

offshore converter, which can effectively limit the capacitor discharge current and cable

current, therefore, the diode current can also be limited in a small range. The capacity of

the diodes in the onshore and offshore converter can also be determined based on the

designed protective inductors, which is enough to tolerate the current under faults. On the

other hand, the redundancy of the system under HVDC transmission system faults is

presented as well, which can be realized with the corresponding switchgears and the

control including onshore converter control, offshore converter control, chopper resistor

control, and the wind turbine control. With the presented fault ride-through control, the

wind farm power can still be sent to the grid as much as possible so as to increase the

system efficiency.

Conclusion

102

Chapter 6

Conclusion

6.1 Summary

In this thesis, a DC grid for offshore wind farms is investigated with focus on the

design and control. The DC grid for offshore wind farms is different from the existing

offshore wind farm. In the DC grid, the wind turbine output is DC not AC, and the wind

turbine can be directly integrated into the DC collection system. Besides, the offshore

converter in the DC grid is a DC/DC converter to step up the collection level DC voltage

to the transmission level DC voltage. Owing to the mature technology for the wind farm

with AC grid, the DC grid structure for the offshore wind farm is similar to the wind farm

structure with AC grid. The wind turbines in the offshore wind farm are connected with a

few clusters in the DC distribution system. And then, these clusters are collected at the

offshore station and the DC level is stepped up by the offshore converter to the

transmission level. Through the HVDC transmission link, the offshore wind farm power

is sent to the onshore, and integrated into the grid. To implement the DC grid for the

offshore wind farm, a few issues should be considered including wind turbine and HVDC

transmission system. Especially, the DC/DC converter for the wind turbine and the

offshore converters are identified as key components.

An IFBTL isolated DC/DC converter is presented for the wind turbine in the DC grid

with the advantages in the aspects of power quality, and EMI for high power applications.

In the three-level configuration, the switches in the converter only take half of the DC bus

voltage, which effectively reduces the dv/dt. A passive filter is inserted into the IFBTL

DC/DC, and improves the performance of the DC/DC converter. As a consequence, the

problem caused by the non-linear characteristics of semiconductor devices can be

overcome by the IFBTL DC/DC converter. In addition, the harmonics and the voltage

103 Conclusion

stress of the MFT is reduced, which is very significant for the power converter in the

high-power application.

Two types wind turbine configurations are presented. One is with the three-level

NPC converter as the generator-side converter and the IFBTL DC/DC converter as the

grid-side converter, where the generator is controlled by the three-level NPC converter

for the optimal power track, and the DC-link voltage in the wind turbine is kept by the

IFBTL DC/DC converter. The other one is with the uncontrollable diode rectifier as the

generator-side converter for AC/DC conversion, and the IFBTL DC/DC converter as the

grid side converter, where the generator is regulated by the IFBTL DC/DC converter for

the optimal power track.

A HVDC transmission system is presented for the DC grid, where the offshore

converter is used for DC/DC conversion. In the HVDC transmission system, the MMC is

used for the onshore converter with the features of modularity and scalability. Especially,

the high number of levels in the MMC enables a great reduction in the device average

switching frequency without compromise of power quality. Furthermore, an inductor is in

series with the distributed energy storage capacitors in each arm to reduce the damage

under faults. The corresponding modulation and control for the onshore converter is

presented. A DC/DC converter based on MMC is also presented for the offshore

converter, which is composed with a three-level MMC, medium frequency transformer,

and diode rectifier. The corresponding control for the offshore converter is also

introduced.

The performance of the DC grid under AC grid faults is presented. During the AC

grid voltage dip, the overvoltage situation may occur at the HVDC transmission system

with the normal control of the DC grid, which may damage the DC grid. A fault ride-

through control is introduced for the DC grid, where the normal control in the DC grid

for wind turbine and HVDC transmission system is improved. With the presented fault

ride-through control, the possible overvoltage situation at the DC grid is avoided, and the

redundant power in the wind farm will be stored in the wind turbine mechanical system.

As a consequence, the wind turbine speed will be increased. Owing to the big inertia of

the wind turbine and the pitch angle control system, the wind turbine speed can be

effectively limited.

The DC cable fault in the HVDC transmission system is harmful for the DC grid,

which may damage the system. The performance of the DC grid under DC cable faults is

studied and analyzed, where a few fault distances are conducted. During the fault, a huge

current may be caused, which may flow through the diodes in the onshore and offshore

converters and damage the converters. In order to protect the system under faults, the

protective inductor is presented and designed, which can limit the cable current and

Conclusion

104

protect the onshore and offshore converters under faults. Moreover, the redundancy of the

HVDC transmission system under faults is presented. Under the action of the switchgears,

the faults parts of the HVDC transmission system can be isolated, and the HVDC

transmission system can be still operated with the healthy converters and cable. The

control of the DC grid under HVDC transmission system faults is proposed. The healthy

onshore converter still keeps the transmission level voltage constant, and the healthy

offshore converter keeps the collection level voltage constant under faults. A chopper

resistor system at the offshore converter may be active to cost the more power at the

collection level and limit the collection level voltage. During the fault, the control for the

wind turbines will be changed, where a power limiter will be inserted into the controller.

A centralized power limit value will be regulated and sent to each wind turbine based on

the HVDC transmission system power. With the proposed fault ride-through control for

the HVDC transmission system, the wind farm power can be sent to the on land as much

as possible under faults, which can effectively improve the system efficiency.

6.2 Proposals for Future Work

As to the DC grid for offshore wind farms, the layout of the wind farm with DC grid

is very important. The layout of the wind farm and the collection level voltage value are

related to the system cost and loss, which needs further investigation.

For the DC/DC converter in the wind turbine, the modular multilevel technology

may be used for the DC/DC converter so as to improve the power quality and increase the

converter efficiency, which is very important. In addition, the possible voltage fluctuation

at the DC-link of the wind turbine may affect the wind turbine operation under system

faults. In order to effectively keep the DC-link voltage, even under the grid faults, the

DC/DC converter associated with its control must be investigated further and limit the

DC-link voltage fluctuation under faults.

For the DC/DC converter at the offshore station, the frequency of the transformer

and the modulation strategy for the DC/DC converter may be researched. A further study

may be needed to improve the converter performance and control, so as to enhance the

HVDC system reliability.

The HVDC transmission system fault is considered, and the corresponding

protection method, redundancy, and the operation and control of the DC grid under faults

is presented. The fault located at the DC collection level and the wind turbine system may

105 Conclusion

be focused in future. The performance of the DC grid under collection system faults and

wind turbine faults may be studied and analyzed. And then, the corresponding protection,

redundancy, and control strategy may be studied so as to improve the system

performance.

In case of super DC grid system, the system layout, the converter characteristic, the

DC grid performances under faults, and the system protection may be needed to be

studied, as well as the DC grid protection and operation under faults. In addition, the

interaction between the DC grid and the power system is very important and worth

researching in future.

References

106

References

[1] Z. Chen, Y. Hu and F. Blaabjerg, “Stability improvement of induction generator-

based wind turbine systems,” Renewable Power Generation, IET. Vol. 1, pp.81-93,

March 2007.

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Appendix A

116

Appendix A

5 MW Wind Turbine System Parameters

TABLE A 5 MW Wind Turbine system Parameters

Parameters Value

Wind turbine rated power (MW) 5

Rotor diameter (m) 126

Rotating speed (r/m) 6.9~11.94

Nominal wind speed (m/s) 11.4

Generator Rated power (MW) 5

Stator rated line voltage (kV) 3

Rated frequency (Hz) 20

Number of pole pairs 100

Stator winding resistance (p.u.) 0.001

Unsaturated induction Xd (p.u.) 0.15

Unsaturated induction Xq (p.u.) 0.1

Magnetic strength (p.u.) 1

Generator inertia (s) 0.84

Equivalent wind turbine inertia (s) 5.54

Shaft stiffness K (p.u.) 2.15

Shaft damping D (p.u.) 0.015

117 Appendix B

Appendix B

2.5 MW Wind Turbine System Parameters

TABLE B 2.5 MW Wind Turbine System Parameters

Parameters Value

Wind turbine power (MW) 2.5

Rotor diameter (m) 100

Rotating speed (r/m) 9.6~15.5

Gear ratio 26:1

Generator power (MW) 2.5

Rated line-to-line voltage (kV) 4

Frequency (Hz) 40

Number of pole pairs 6

Stator winding resistance Rs (p.u.) 0.0038

Unsaturated induction Xd (p.u.) 0.25

Unsaturated induction Xq (p.u.) 0.62

Magnetic strength (p.u.) 1.1

Capacitors Cg (μF) 150

Capacitors Ci1 and Ci2 (mF) 6.8

Turn ratio of the MFT 1:7

Inductance Ls (mH) 0.4

Capacitor Cs (μF) 15

Inductance Ld (mH) 1

Capacitor Co (μF) 330

Cable inductance Lc (mH/km) 0.5

Cable resistance Rc (Ω/km) 0.15

Cable length (km) 10

DC network voltage Vo (kV) 40

Switching frequency (kHz) 2

Appendix C

118

Appendix C

Experimental Circuit Parameters

TABLE C IFBTL DC/DC Converter Experimental Parameters

Parameters Value

Capacitors Ci1 and Ci2 (uF) 330

Inductance Ls (mH) 0.4

Capacitor Cs (μF) 3

Turn ratio of the MFT 1:2.6

Inductance Ld (mH) 0.4

Capacitor Co (μF) 1000

DC network voltage Vo (V) 250

DC network inductance (mH) 0.4

DC network resistance (Ω) 0.1

Switching frequency (kHz) 5

119 Appendix D

Appendix D

Cable Parameters

The frequency dependent phase model is applied as the simulation model for cable in

PSCAD/EMTDC [109-101]. The cable layout is shown in Fig. A.1, which consists of a

copper core conductor with a cross section of 1200 mm2, a lead sheath, a steel armor, and

insulating layers. All relevant data are listed in Table I, which is based on a cable design

that is assembled from [9], [112] and [113].

ConductorInsulator 1

SheathInsulator 2

Armor

Insulator 322.0 mm

39.0 mm

42.0 mm

47.0 mm

52.0 mm

56.0 mm

Fig. D.1. Dimensions of the cable model

TABLE D Properties of the Cable

Layer Material Thickness

(mm)

Resistivity

(Ω·m)

Rel. per-

mittivity

Rel. per-

meability

Core copper 22 1.68·10-8

- 1

Insulator XLPE 17 - 2.3 1

Sheath Lead 3 2.2·10-7

- 1

Insulator XLPE 5 - 2.3 1

Armor Steel 5 1.8·10-7

- 10

Insulator PP 4 - 2.1 1

Appendix E

120

Appendix E

HVDC Transmission System Parameters

TABLE E HVDC Transmission System Parameters

Parameters Value

Rated power for each positive (negative) converter (MW) 200

DC-link voltage Vn1 and Vn2 (kV) 150

Capacitor Cn1, Cn2, Cf1 and Cf2 (uF) 35.5

Reactor inductor Lf (mH) 16

Reactor resistor Rf (Ω) 0.16

AC Transformer leakage reactance 20 %

Onshore converter switching frequency (Hz) 1950

Offshore converter switching frequency (Hz) 1950

Offshore converter reference frequency (Hz) 200

Documents

Design and Control of a DC Grid for Offshore Wind Farms Final 20121206